I couldn't find this under Phone Tropes
. Essentially, Bob knows that there's something coming up that he would love to get out of, so he arranges for Alice to phone him during the event, pretending that there's an emergency and forcing him to leave.
There's also a variant where it isn't planned - someone just happens to phone Bob during the event, and since nobody else can hear them, Bob pretends that he's being called away on an emergency, often confusing the person on the other end.
Possibly I missed this since it's not necessarily a phone trope, but I couldn't find it.
I can't think of any examples off the top of my head, but I know it's been done lots of times.