This might sound cheesy or stereo-typical, but my favorite phone sex role playing scenario is the following:

I have an appointment at my therapist. I pretend that it is my first consultation.

The therapist is gay and I am a straight guy who has never been with a man. The therapist starts the session in the usual way with a bunch of standard background questions. There are some questions about my childhood and he slowly helps me remember intimate moments that I had with girls and boys.

He guides me to explain how I have always been attracted to boys and secretly admired their young bodies in the change room, I obviously deny every word, but he is persistent and asks me more and more questions of an increasingly sexual nature.

We fast forward to my early days in the workplace. Describing my handsome boss and his equally attractive partner. Certain events unfold and the scene shifts to one of soft lights, gentle romantic music and ever increasing sensuality and sexuality. You see, this is where the game really starts and whatever is said next will determine success or failure.

I don’t want to give away all my secret because I don’t want to spoil the fun. But at this point of the scenario I am naked with my boss and his partner. He is massaging my shoulders while she is dancing in front of me in a sensual way.

Events unfold and we are all naked sensually touching each other in a pool of lustful desire. His voice fades, softer, slower and more erotic. He is describes how he (as the therapist) is now also touching me while he is asking me to describe how my boss is touching me and his partner touching him…

There is just something about somebody describing what they do to me while I describe something that I am doing to another person that does it for me.
It just adds a whole new layer.
I think I might just need some therapy, excuse me please, I need to go make a call.