You can call for free with IP telephone to her mobile. You need broadband connection. I use VoipDiscount and Im calling my GF for free to her mobile.
I have a wirerless DIP phone so i can call her from anywere in my house or my garden.
I have been using their service for +2 years and no problems.
I pay 10 euro and can call for free 120 days. After 120 days they charge 1 cent per minute. There is a limit how long you can call. I think its 330 minutes per 10 days. Half hour per day. But if you dont call one day you can call 1 hour next day and so one.
check it out:
VoipDiscount | Free Calls and SMS
repeter