Not using a phone - simply a phone ringing and no one wanted to own up. It was implied that the phone was in the cabin but may well have been in the hold.
If so then it would the problem already anticipated elsewhere in the forums - that people will forget to turn off the phone before shoving it in the hold bag.
BBC has the story.
A British Airways flight from Heathrow to New York has been turned back after a mobile phone was heard ringing at the back of the plane. No-one on board the plane admitted owning the phone, banned under current security restrictions, so flight BA179 returned to the London airport.
The scare came as the Department for Transport downgraded the UK terror threat from critical to severe.