It sounds like this girl's behavior is very troubling as it would be for most girlfriends. The fact that she won't take a hint is more frustrating. The person able to change this is him. If this is a relationship issue he needs to address this with her - once and for all. If he is inconsistent then she won't believe that her efforts are useless. No one is going to be that persistent unless there is hope. He can put a stop to it.
As far as the fantasizing he I guess is telling you all this. That adds to your insecurity. Actually dreams are the result of a subconscious process and doesn't mean he loves her. Fantasizing however is conscious. That he has control over. If he is fantasizing that is an issue. These fantasies are a choice. This is something that is going to take a lot of positive communication on both your parts. She is not a threat in a healthy relationship but this fantasizing is troublesome. Try to find out what part he is playing in this scenario.
I think the key is with him. Tell him first to put an end to the calls even if it means changing the number. Ask him to explain to you why he is doing this and help him understand how it makes you feel to have him fantasizing about someone else. Work on this together.
If this has been helpful press accept