This was actually pretty funny:
It happened in practice, not a gig. My old band was playing through some stuff one night when the guitar palyers girlfriend called. Anyway, his phone vibrated in his pocket so he stopped playing right in the middle of the damn song!

He runs out of the room to talk to her.
We all yelled at him but it didn't do any good. So I got on the mic and the drummer picked up the guitar and the singer got on the drums and we started playing an obnoxiously loud, chromatic, death metal rif. Double bass srums, thundering low B bass and guitar (the guitard had a seven string). And I was screaming cookie monster type vocals into the mic. I was making the lyrics up as I went along but it was something like...
"Get off the phone! Get off the phone! Get off the phone! Your girlfriend calls and you drop your d_ _ _. I F_ _ _ _ _ _ g hate you, you make me sick! You really suck and she ruined our band. We would be better off if you would just f_ _ _ your hand...GET OFF THE PHONE!!!! GET OFF THE PHONE!!!" Of course we were all good friends and none of his was meant maliciously. We always gave each other a hard time about aqnything and everything. And we picked on him about her all the time.
But we were all laughing so hard that it was difficult to maintain the playing. And screaming in that low, death metal style kills my throat, so I started coughing all over the place. That just madethe other guys laugh harder. And the whole time the guitard is standing outside of our practice space giving us all the finger and throwing rocks at us.
It was hilarious. It bacame a total inside joke too. Anytime after that, when she would call him; regardless of where we were or what we were doing; we would break into the "Get off the phone" song.