Did you say anything? If not, play dumb, say your friend called and told you they called, but you didn't get a call/have no idea what happened. I used to have a job on the phone all day. People call while on the toilet, and we'd hear their business, the flush, hand washing (if they weren't nasty), etc... hated it! Call them, give them an excuse, they'll tell themselves what they heard is not what they think they heard and everybody will be happy.