Hi
You don’t say how old this person is or if you engage with them in person.
I don’t have enough information.
I’m going to guess that every time you talk to this person, this person believes that you enjoy the conversation as much as they so they want more.
The cruelest part of autism is that people with ASD miss signals, although they have all the wants, needs, desires and feelings as neuro typical people. Their feelings get hurt, too.
Don’t sugar coat, you need to be direct but kind.
Perhaps you might say, “Your phone calls are too much. You are a good person, but I am not the person you want me to be. Please do not phone me anymore, okay?” - Or whatever fits.
Your friend will hurt. Gosh, it hurts me writing this, but again, many people with ASD don’t pick up on inferences or subtleties. Ghosting won’t work. They are very literal.