Boost Mobile requires a special iDen phone.

Jolt appears to be a MVNO on Cingular. This means that the coverage is both 850 MHz and 1900 MHz. I don't see a SIM-only deal but using a tri-band (900/1800/1900) phone with that SIM would be like using a 850/1800/1900 phone on TIM. It would work in some places but not in many others. You might want to order their cheap phone. Jolt's "free" international calling looks pretty good but I seriously doubt that calls to mobiles are included in that rate. The other alternative is to use another prepaid and a call-through service like gorillamobile.com or voicestick.com.