Turn it into a joke. Tell her something funny like "oh I'm sorry I'm speaking like I'm having a rock in my mouth" or "I'm trying to stop it but I can't, maybe my tongue is doing dancing all on its own I don't know", she would be happy to hear that you are not shy about it and you make her smile and show confidence to her. Also, you won't feel dumb after the talk if you stop being ashamed of it. Secondly ask her to go chatting instead of talking or to talk less time on the phone. I hope she will understand you and your stuttering won't get in your way