The very first assignment I was given in my very first acting class was to create a phone conversation. We were to write a script for both sides of the conversation. We would memorize our own script, and, using a prop phone, perform it for the rest of the class. All that week I watched people on the phone in real life. I made specific efforts to include those nuances I had observed into my peformance. The result? I got good marks.
More than one of my fellow students missed said nuances, however. Perhaps this is why our acting professor chose this as our very first assignment.
Indeed, telephone conversations wherein the other party is not heard on stage seem to be a problem not only for students, but also to more experienced actors.
The most frequent transgression seems to be cutting off the "other party" entirely too quickly. A caller on the other end is able to magically present their name, pleasantries, and the purpose of their call within 3 seconds. (Judging by many performances.) Often, a whole phone conversation on stage flies by at lightning speed. I think the reason this happens is very clear; people fear silence on stage.
It is a very natural tendency, particularly with newer actors, to want to fill every moment with some sort of noise. It is an understandable, but deadly temptation. Give your imaginary caller time to get through what he has to say. If it helps, write out a script for what they are most likely saying on the other end, and give them time to say it. Silence with a purpose is still acting. Listening on the phone is one such appropriate time for silence.
That is not to say that during said silence, you should do nothing. Your performance continues. Despite this, almost as frequent as the "cutting off" problem is "Phone Phreeze".
What is "Phone Phreeze"? It is my cutsey term for a very real obstacle faced by many actors. Frequently when using a phone on stage the actor will stand or sit, perfectly still. The eyes are transfixed either on some unchanging point on the set, or even worse, the back of the house. While some scripts may require such a reaction, in general this is not how people behave on the telephone.
Pay attention to yourself or someone else the next time you take a phone call. Do you stand at attention and lock up? Or do you look around, laugh, nod, occasionally interrupt the other person by mistake, fiddle with something on a nearby table, and the like? My guess is the latter. Therefore unless there is a plot driven reason to do none of those things, remember to incorporate at least some of them into your stage phone conversation.
With a little attention and extra effort, there is no need to phone it in with phones.