Well, the Treo phones are Palm and can be used on any GSM network carrier. Then, Cingular & Verizon have branded ones. (I have an unbranded & unlocked one & decided to use T-mobile). The Cingular ones can be used on with a different GSM carrier if you ever decide to cancel service with them but I don't know if the same applies for the Verizon branded one. However, I do know that an unbranded Treo (or branded with something other than Verizon) can not be used on Verizon, which stinks. ... Verizon doesn't use SIM cards I think.