I don't want this girl I'm talking to to have phone calls with her ex
Why won't you let her have phone calls with her ex?
I think that they might try to rekindle their love again
You have no right to tell the girl what to do. If she wants to talk to her ex, that's her business.
This sucks.
Maybe you shouldn't be with this girl if this upsets you so much. In any case, you shouldn't control who she talks to.