My good friend and solid wing last year once had a girl use her boyfriend's phone (who was talking to some other people behind her) to take down his number. It was one of the funniest things id seen.
He never had a special bf buster line, the guy just showed no reaction and move topic asap. kinda like 'yeh, cool' and continue with another/previous topic. which for him seemed to be good enough as the whole mention of a boyfriend thing is to provoke a reaction.