The competition challenges members of the public to come up with the best party piece. People can then go into one of the phone boxes and film themselves performing their trick in order to enter the competition.
The viral campaign simply shows some examples of people performing their party piece. In one, a man drums the William Tell overture on his teeth. A second shows a man making his eyes bulge out of his head.
The comedian Al Murray will judge the entries as his Pub Landlord character.
Murray's Pub Landlord is also the subject of a viral, in which he is shown downing a pint of beer in less than three seconds before performing a tongue twister.
The films end with BT challenging consumers to enter the competition if they "think they can do better". It directs them to BT's blue phone boxes, where they can enter, and also gives a web address for further information (whatsyourpartypiece.com). The winners can host a party paid for by BT.
The virals will be sent to a list of 90,000 18- to 24-year-olds, who are expected then to forward them to friends they think will enjoy the virals.
The campaign was written by Laurie Smith and art directed by Matt Pam. PHD planned and bought media for the campaign, which kicks off this week.
Charles Inge, the creative partner at CHI, said: "It is great to be working with BT on an idea that goes beyond traditional media channels."
CHI secured a place on the BT roster in March. Its initial brief was to promote the blue kiosks, but this was expanded to handling the company's BT Yahoo! account, with an estimated budget of £12 million.