Okay I have been wondering this as well and from what I've researched, if you have AT&T or T-Mobile you will need to have a phone that operates on a GSM network. If you are with Sprint or Verizon, you need a phone that will operate on a CDMA network. In the US, Sprint, Verizon and US Cellular use CDMA. AT&T and T-Mobile use GSM. I am an AT&T user myself, which means I will be purchasing the unlocked T-Mobile version and paying in full up front so I can easily pop my AT&T sim card into it and voila I have my new phone with my number/contacts/everything in it. If you are not in the US, you should check to see what network your service runs on and make sure to get the unlocked version that is compatible.