Other good ones:
"Oh hold on, I'm getting another call!" also add, if necessary "This is a new phone and I'm not used to it, so if I lose you..."
"Was that the door? I'm waiting for delivery. Gotta go!"
"Sorry to be rude but I'm waiting for a call back from the doctor."
"I think we have a crossed line. Can you hear that? I'll call you back."
"Yes, dear! Coming! Sorry, you know how it is. Got to go!"
"Think I'm about to go into a tunnel so may lose..."
or my favourite:
"Oh ****! My battery's almost dea -"
If all else fails:
"Hello? Hello? HEL-LO? Is anyone there?"