Insert "Intercut phone conversation" before the dialogue of the first person who is on the phone. This will indicate that both actors will be onscreen or intercut during the phone conversation.
Use the initials "V.O." which stand for "Voice Over" if you only want only one actor to be seen but both actors to be heard. For example, insert "V.O" after the character that you want to be heard but not seen each time he has dialogue. Then write the voice over dialogue you want the character to say.
Use a couple different methods when you want only one actor to be seen and heard while on the phone. Write description about the one actor you want onscreen. An example description would be something like "Anne picks up the ringing telephone." Follow it with just Anne's dialogue. You could even write "Anne (into phone)" before each of her lines. Or insert "(beat)" after every one of her pieces of dialogue into the phone. Inserting the "beat" in parentheses indicates that the character is reacting to what she is hearing to on the phone.