The iPhone allows audio conferencing, call holding, call merging, caller ID, and integration with other cellular network features and iPhone functions. For example, if music is playing when a call is received, the music fades out, and fades back in when the call has ended. The proximity sensor shuts off the screen and touch-sensitive circuitry when the iPhone is brought close to the face, both to save battery and prevent unintentional touches. The iPhone does not support video calling orvideoconferencing on versions prior to the fourth generation, as there is only one camera on the opposite side of the screen.  The iPhone 4 supports video calling using either the front or back camera over Wi-Fi, a feature Apple calls FaceTime. The first two models only support voice dialing through third-party applications.  Voice control, available only on the iPhone 3GS and iPhone 4, allows users to say a contact's name or number and the iPhone will dial. 

The iPhone includes a visual voicemail (in some countries)  feature allowing users to view a list of current voicemail messages on-screen without having to call into their voicemail. Unlike most other systems, messages can be listened to and deleted in a non-chronological order by choosing any message from an on-screen list.

A music ringtone feature was introduced in the United States on September 5, 2007. Users can create custom ringtones from songs purchased from the iTunes Store for a small additional fee. The ringtones can be 3 to 30 seconds long from any part of a song, can fade in and out, pause from half a second to five seconds when looped, or loop continuously. All customizing can be done in iTunes, or alternatively with Apple's GarageBand software 4.1.1 or later (available only on Mac OS X) or third-party tools.

The layout of the music library is similar to that of an iPod or current Symbian S60 phones. The iPhone can sort its media library by songs, artists, albums, videos, playlists, genres, composers, podcasts, audiobooks, and compilations. Options are always presented alphabetically, except in playlists, which retain their order from iTunes. The iPhone uses a large font that allows users plenty of room to touch their selection. Users can rotate their device horizontally to landscape mode to access Cover Flow. Like on iTunes, this feature shows the different album covers in a scroll-through photo library. Scrolling is achieved by swiping a finger across the screen. Alternatively, headset controls can be used to pause, play, skip, and repeat tracks. On the iPhone 3GS, the volume can be changed with the included Apple Earphones, and the Voice Control feature can be used to identify a track, play songs in a playlist or by a specific artist, or create a Genius playlist.

The iPhone supports gapless playback. Like the fifth-generation iPods introduced in 2005, the iPhone can play digital video, allowing users to watch TV shows and movies in widescreen. Double-tapping switches between widescreen and fullscreen video playback.

The iPhone allows users to purchase and download songs from the iTunes Store directly to their iPhone. The feature originally required a Wi-Fi network, but now  can use the cellular data network if one is not available.

The iPhone includes software that allows the user to upload, view, and e-mail photos taken with the camera. The user zooms in and out of photos by sliding two fingers further apart or closer together, much like Safari. The Camera application also lets users view the camera roll, the pictures that have been taken with the iPhone's camera. Those pictures are also available in the Photos application, along with any transferred from iPhoto or Aperture on a Mac, or Photoshop on a Windows PC.

Internet access is available when the iPhone is connected to a local area Wi-Fi or a wide area GSM or EDGE network, both second-generation (2G) wireless data standards. The iPhone 3G introduced support for third-generation UMTS and HSDPA 3.6,  but not HSUPA networks, and only the iPhone 3GS and iPhone 4 support HSDPA 7.2.  AT&T introduced 3G in July 2004,  but as late as 2007, Steve Jobs stated that it was still not widespread enough in the US, and the chipsets not energy efficient enough, to be included in the iPhone.  Support for 802.1X, an authentication system commonly used by university and corporate Wi-Fi networks, was added in the 2.0 version update. 

By default, the iPhone will ask to join newly discovered Wi-Fi networks and prompt for the password when required. Alternatively, it can join closed Wi-Fi networks manually. The iPhone will automatically choose the strongest network, connecting to Wi-Fi instead of EDGE when it is available. Similarly, the iPhone 3G, 3GS and 4 prefer 3G to 2G, and Wi-Fi to either.  Wi-Fi, Bluetooth, and 3G (on the iPhone 3G, 3GS, and 4) can all be deactivated individually. Airplane mode disables all wireless connections at once, overriding other preferences. However, once in Airplane mode, one can explicitly enable Wi-Fi and/or Bluetooth modes to join and continue to operate over one or both of those networks while the cellular network transceivers remain off.

The iPhone 3GS has a maximum download rate of 7.2 Mbit/s. Furthermore, email attachments as well as apps and media from Apple's various stores must be smaller than 20 MB to be downloaded over a cellular network. Larger files, often email attachments or podcasts, must be downloaded over Wi-Fi (which has no file size limits). If Wi-Fi is unavailable, one workaround is to open the files directly in Safari.

Safari is the iPhone's native web browser, and it displays pages similar to its Mac and Windows counterparts. Web pages may be viewed in portrait or landscape mode and the device supports automatic zooming by pinching together or spreading apart fingertips on the screen, or by double-tapping text or images. The iPhone supports neither Flash nor Java.Consequently, the UK's Advertising Standards Authority adjudicated that an advertisement claiming the iPhone could access "all parts of the internet" should be withdrawn in its current form, on grounds of false advertising. In a rare public letter in April 2010, Apple CEO Steve Jobs outlined the reasoning behind the absence of Flash on the iPhone (and iPad). The iPhone supports SVG,CSS, HTML Canvas, and Bonjour.

The maps application can access Google Maps in map, satellite, or hybrid form. It can also generate directions between two locations, while providing optional real-time traffic information. During the iPhone's announcement, Jobs demonstrated this feature by searching for nearby Starbucks locations and then placing a prank call to one with a single tap. Support for walking directions, public transit, and street view was added in the version 2.2 software update, but no voice-guided navigation. The iPhone 3GS and iPhone 4 can orient the map with its digital compass. Apple also developed a separate application to view YouTube videos on the iPhone, which streams videos after encoding them using the H.264 codec. Simple weather and stock quotes applications also tap in to the Internet.

iPhone users can and do access the Internet frequently, and in a variety of places. According to Google, in 2008, the iPhone generated 50 times more search requests than any other mobile handset. According to Deutsche Telekom CEO René Obermann, "The average Internet usage for an iPhone customer is more than 100 megabytes. This is 30 times the use for our average contract-based consumer customers." Nielsen found that 98% of iPhone users use data services, and 88% use the internet. In China, the iPhone 3G and iPhone 3GS were built and distributed without Wi-fi.

With the introduction of the Verizon iPhone in January 2011, the issue of using internet while on the phone has been brought to the public's attention. Under the two US carriers, internet and phone could be used simultaneously on ATT networks, whereas the Verizon networks only support for the use of each separately.

For text input, the iPhone implements a virtual keyboard on the touchscreen. It has automatic spell checking and correction, predictive word capabilities, and a dynamic dictionary that learns new words. The keyboard can predict what word the user is typing and complete it, and correct for the accidental pressing of keys near the presumed desired key.[157] The keys are somewhat larger and spaced farther apart when in landscape mode, which is supported by only a limited number of applications. Touching a section of text for a brief time brings up a magnifying glass, allowing users to place the cursor in the middle of existing text. The virtual keyboard can accommodate 21 languages, including character recognition for Chinese.  Alternate characters with accents can be typed from the keyboard by pressing the letter for 2 seconds and selecting the alternate character from the popup.  The 3.0 update brought support for cut, copy, or pasting text, as well as landscape keyboards in more applications.  pports HTML e-mail, which enables the user to embed photos in an e-mail message. PDF, Word, Excel, and Powerpoint attachments to mail messages can be viewed on the phone.  Apple's MobileMe platform offers push email, which emulates the functionality of the popular BlackBerry email solution, for an annual subscription. Yahoo! offers a free push-email service for the iPhone. IMAP (although notPush-IMAP) and POP3 mail standards are also supported, including Microsoft Exchange  and Kerio Connect.  In the first versions of the iPhone firmware, this was accomplished by opening up IMAP on the Exchange server. Apple has also licensed Microsoft ActiveSync and now  supports the platform (including push email) with the release of iPhone 2.0 firmware. The iPhone will sync e-mail account settings over from Apple's own Mailapplication, Microsoft Outlook, and Microsoft Entourage, or it can be manually configured on the device itself. With the correct settings, the e-mail program can access almost any IMAP or POP3 account. 

Text messages are presented chronologically in a mailbox format similar to Mail, which places all text from recipients together with replies. Text messages are displayed in speech bubbles (similar toiChat) under each recipient's name. The iPhone has built-in support for e-mail message forwarding, drafts, and direct internal camera-to-e-mail picture sending. Support for multi-recipient SMS was added in the 1.1.3 software update.  Support for MMS was added in the 3.0 update, but not for the original iPhone  and not in the U.S. until September 25, 2009.

At WWDC 2007 on June 11, 2007, Apple announced that the iPhone would support third-party "web applications" written in Ajax that share the look and feel of the iPhone interface. On October 17, 2007, Steve Jobs, in an open letter posted to Apple's "Hot News" weblog, announced that a software development kit (SDK) would be made available to third-party developers in February 2008. The iPhone SDK was officially announced and released on March 6, 2008, at the Apple Town Hall facility. It is a free download, with an Apple registration, that allows developers to develop native applications for the iPhone and iPod Touch, then test them in an "iPhone simulator". However, loading an application onto a real device is only possible after paying an Apple Developer Connectionmembership fee. Developers are free to set any price for their applications to be distributed through the App Store, of which they will receive a 70% share. Developers can also opt to release the application for free and will not pay any costs to release or distribute the application beyond the membership fee. The App Store was launched with the release of iOS 2.0, on July 11, 2008. The update was free for iPhone users; owners of older iPod Touches were required to pay US$10 for it.

Once a developer has submitted an application to the App Store, Apple holds firm control over its distribution. Apple can halt the distribution of applications it deems inappropriate, for example, I Am Rich, a US$1000 program that simply demonstrated the wealth of its user. Apple has been criticized for banning third-party applications that enable a functionality that Apple does not want the iPhone to have: In 2008, Apple rejected Podcaster, which allowed iPhone users to download podcasts directly to the iPhone claiming it duplicated the functionality of iTunes. Apple has since released a software update that grants this capability.

NetShare, another rejected app, would have enabled users to tether their iPhone to a laptop or desktop, using its cellular network to load data for the computer. Many carriers of the iPhone later globally allowed tethering before Apple officially supported it with the upgrade to the iOS 3.0, with AT&T Mobility being a relative latecomer in the United States. In most cases, the carrier charges extra for tethering an iPhone.

Before the SDK was released, third-parties were permitted to design "Web Apps" that would run through Safari. Unsigned native applications are also available for "jailbroken" phones. The ability to install native applications onto the iPhone outside of the App Store is not supported by Apple, the stated reason being that such native applications could be broken by any software update, but Apple has stated it will not design software updates specifically to break native applications other than those that perform SIM unlocking.

As of January 2011, Apple has passed 10 billion app downloads.

The iPhone can enlarge text to make it more accessible for vision-impaired users, and can accommodate hearing-impaired users with closed captioning and external TTY devices. The iPhone 3GS also features white on black mode, VoiceOver (a screen reader), and zooming for impaired vision, and mono audio for limited hearing in one ear. Apple regularly publishes Voluntary Product Accessibility Templates which explicitly state compliance with the US regulation "Section 508".

On July 7, 2011, Apple announced that in new versions of the iOS software, a security problem would be fixed. Users who install their own software without Apple's protections can have their information stolen or unwittingly download malware by clicking on certain PDF files. This discovery of "critical weaknesses" was reported by the German Federal Office for Information Security. 

On July 15, 2011, Apple released iOS 4.3.4 (4.2.9 for CDMA iPhone) in response to JailbreakMe 3.0 from comex (a developer). It uncovered a userland PDF exploit that is fixed in this update.

On July 25, 2011, Apple released iOS 4.3.5 (4.2.10 for CDMA iPhone) to fix a security vulnerability with certificate validation. 

Apple has filed more than 200 patent applications related to the technology behind the iPhone. 

LG Electronics claimed the iPhone's design was copied from the LG Prada. Woo-Young Kwak, head of LG Mobile Handset R&D Center, said at a press conference, "We consider that Apple copied Prada phone after the design was unveiled when it was presented in the iF Design Award and won the prize in September 2006."

On September 3, 1993, Infogear filed for the U.S. trademark "I PHONE" and on March 20, 1996 applied for the trademark "IPhone". "I Phone" was registered in March 1998, and "IPhone" was registered in 1999. Since then, the I PHONE mark had been abandoned. Infogear's trademarks cover "communications terminals comprising computer hardware and software providing integrated telephone, data communications and personal computer functions" (1993 filing), and "computer hardware and software for providing integrated telephone communication with computerized global information networks" (1996 filing). Infogear released a telephone with an integrated web browser under the name iPhone in 1998. In 2000, Infogear won an infringement claim against the owners of the iphones.com domain name. In June 2000, Cisco Systems acquired Infogear, including the iPhone trademark. On December 18, 2006, they released a range of re-branded Voice over IP (VoIP) sets under the name iPhone.

In October 2002, Apple applied for the "iPhone" trademark in the United Kingdom, Australia, Singapore, and the European Union. A Canadian application followed in October 2004, and a New Zealand application in September 2006. As of October 2006, only the Singapore and Australian applications had been granted. In September 2006, a company called Ocean Telecom Services applied for an "iPhone" trademark in the United States, United Kingdom and Hong Kong, following a filing in Trinidad and Tobago. As the Ocean Telecom trademark applications use exactly the same wording as Apple's New Zealand application, it is assumed that Ocean Telecom is applying on behalf of Apple. The Canadian application was opposed in August 2005, by a Canadian company calledComwave who themselves applied for the trademark three months later. Comwave has been selling VoIP devices called iPhone since 2004.

Shortly after Steve Jobs' January 9, 2007 announcement that Apple would be selling a product called iPhone in June 2007, Cisco issued a statement that it had been negotiating trademark licensing with Apple and expected Apple to agree to the final documents that had been submitted the night before. On January 10, 2007, Cisco announced it had filed a lawsuit against Apple over the infringement of the trademark iPhone, seeking an injunction in federal court to prohibit Apple from using the name. More recently, Cisco claimed that the trademark lawsuit was a "minor skirmish" that was not about money, but about interoperability.

On February 2, 2007, Apple and Cisco announced that they had agreed to temporarily suspend litigation while they held settlement talks, and subsequently announced on February 20, 2007, that they had reached an agreement. Both companies will be allowed to use the "iPhone" name in exchange for "exploring interoperability" between their security, consumer, and business communications products.

The iPhone has also inspired several leading high-tech clones, driving both Apple's popularity and consumer willingness to upgrade iPhones quickly.

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