Replying to Iphone, AT&T and Verizon

The other day, in a freak instance of horrible luck, I was driving with my windows opened when a random pot hole caused my car to jolt and toss my phone out of the window. It hit the road and was immediately run over by a mack truck. Im looking into cancelling my verizon plan early to buy an Iphone on the family plan, but the only problem is that my GF has Verizon. She is probably the only person I call. Does it jack up her bill if an ATT user calls/texts her? Is there any way around this, other than getting a droid with VZ service?