Should I HAVE to use the same cell phone carrier as my ex so as not to cost him money???

I have Alltel and he has Cingular gophone. Well, it costs him money to call my phone. We share custody of the kids so communication is VITAL. But he gets mad because he says me not having cingular is costing him money. It's free for me to call him because I put hi in the "my Circle" plan on my contract. Should I really HAVE to also pay for a go phone just to save him money?