The sign for TELEPHONE was one of the first signs that my son did very well and he became obsessed with it. It’s an easy sign to do because it’s a “natural sign” that looks an awful lot like the word it’s trying to replace. You just bring your hand up to your ear like you are talking on a telephone with thumb and pinky finger extended. Don’t expect your baby to do the sign perfectly though, he’ll probably just bring his hand up and cover his ear, but that’s a pretty good sign approximation.
Once my baby had learned the sign, there were suddenly phones everywhere! He’d ask for our portable model and carry it all over the house pretending to talk on it. It got to the point where I dug up an old corded spare phone which we keep for emergencies and gave that to him so we could have of it. Once that association was made, he’d even sign it out when the phone rang. He wasn’t so sure what to do when grandma got on the phone though! A bit puzzled we coxed a few murmurs out of him though and eventually taught him how to do a version of “Hi” and then do his animal sounds as he grew more verbal. Even at 16 months, he’d still do the sign when he heard one ring on television.