The ad, by Abbot Mead Vickers BBDO with media by Maxus, is the latest in the series that follows the lives of Adam and Jane.
This time, a heavily pregnant Jane is planning her wedding when she suddenly goes into early labour.
With the new arrival, the couple no longer have the time to organise their wedding, so viewers will be led to a dedicated Facebook page where they can watch videos of Jane contemplating her choices of dress, car and first dance, and select their favourites.
The options with the most votes will be used in the next ad, to be shown in April. BT will choose two voters at random to star in the ad as wedding guests, the first time members of the public have been given a chance to feature in a BT ad.
It is the second time BT has asked the public to determine the lives of Adam and Jane. In August last year, 1.6 million people took part in the Facebook vote, which decided Jane was pregnant.
David James, marketing director, consumer, BT Retail, said: "This is an important moment in a much loved storyline which has been running for nearly six years.
"There was an overwhelming response to the pregnancy vote, and it seems only right that Adam and Jane fans should have a chance to be a part of the couple's big day, as they finally tie the knot."
The ad's copywriter was Phill Martin, the art director was Brian Campbell and the director was Declan Lowney. The production company was His London, with post production by The Mill and audio post production by Wave.