The Phone-a-Friend lifeline was one of the original three lifelines on the show, when it first aired in the UK on 4 September, 1998. The UK version used it for its entirety; however, the U.S. show has used it only until early 2010, when it was discontinued because of an increasing trend in contestants’ friends using Internet search engines to look up the right answer (while this was not necessarily a rules violation, it was contrary to the original intent of this lifeline). This lifeline was sponsored by AT&T for the U.S. primetime run from 1999-2002, the first season of the syndicated version from 2002-2003, and 2009 primetime revival. On the other hand, whereas the U.S. version discontinued this lifeline, the UK version continued to use this lifeline, but instead changed how the lifeline worked; instead of leaving the selected friends at home, the contestant selects their friends ahead of time, and the show brings them backstage and places them in isolated soundproof booths, where they will not see or hear anything that goes on in the main studio until they are called for assistance, and they will not have any access to the internet.
In all versions of the game that have this lifeline, a contestant must give the show a list of three or five friends before gameplay begins that can be called (they may not choose any other friends during gameplay). These friends will be called just as a contestant begins gameplay in order to remind them to be on stand-by and to keep the line as open as possible. On live shows, the show may bring said friends to a backstage location somewhere else in the studio and keep them away from telephones, TVs, and access to outside parties, in order for them to not know the contents of the questions a contestant is facing beforehand.
In the U.S., UK, and other countries' Clock Formats, when the contestant chose to use the lifeline, the clock would be stopped. Once the call was finished, the host would give a brief explanation, and then the clock would start again.
A variation of Phone-a-Friend, called Phone-a-Complete Stranger, was used on the former Who Wants to Be a Millionaire – Play It! attraction. Instead of calling someone from the contestant's list of friends, a Disney Cast Member would get called and asked by the host to find a park guest to help the contestant with the question they need help on.
During Star Wars Weekends at Disney's Hollywood Studios, when the attraction had questions asked about Star Wars, the lifeline would be temporarily renamed Phone-a-Stormtrooper. For ESPN The Weekend, it would be replaced with a chance to ask an ESPN expert for assistance.
- Regis Philbin said on the first episode of the U.S. version that Phone-a-Friend was his favorite lifeline.
- On three occasions, a contestant would use the Phone-a-Friend lifeline on the last question, not for help, but to inform their friend that they were about to win the top prize, then eventually do so. The most common example is John Carpenter. The other two are Ralf Schnoor and Enrique Chicote.
- For the 9th season of Kaun Banega Crorepati, the 2017 season of Ai Là Triệu Phú, and Quiz $ Millionaire, they will be using video calls instead of audio calls.
- When Eddie Timanus used the Phone-a-Friend lifeline during his Millionaire run, since he was blind, host Meredith Vieira would read the question and four possible choices to Eddie's friend instead and also remind him of how much time remained on the 30 second clock.
- Since the 2018 revival in the UK version, the friends would be accompanied by a member of the production team to prevent them from cheating.
- In the first episode of the UK version, the Phone-a-Friend lifeline was conducted using a handheld landline telephone. It was only used during Graham Elwell's run.
- When Tko želi biti milijunaš? and BankGiro Miljonairs returned in 2019, the Phone-a-Friend lifeline shortened from 30 to 25 seconds.