I had a relative who was incarcerated recently and wanted to make collect calls to a cellphone. There's a Correctional Billing System that handles that, but you, the cell phone user, prepays upfront for all the calls. At the time they wanted a minimum payment of $250 of prepaid airtime, no less. On top of that, they were the surliest, rudest company I ever dealt with. They did every thing short of calling me a deadbeat because I didn't want to pay $250 for one phone call and treated me like garbage because I wanted to talk to someone that was in jail.
I do know that at some places where they sell calling cards, there is some kind of global calling card you can buy and setup to use on cell phones. My friend was kind of in the same situation and that's what he did. He said the card he bought was like $25.00 plus tax and his girlfriend could call him 5 times. I myself am looking into it and will keep you updated.