You can't use a GSM phone (uses a SIM card) on Sprint which uses the CDMA network. The technologies are incompatible. You could on use it on At&t or T-mobile as long as the phone is unlocked.
Purchased a Motorizr z3 for the wife to use in UK on a recent trip. She bought a sim and minutes on the Lebara network there and it worked fine. She's now returned and I'd like to use it to replace her Motorola 730i on Sprint. If I simply replace the sim card in the Motorizr with the one from the 730i it cannot find the network. How do I activate the Motorizr on the Sprint network with the phone number from the 730i?