If you have a GSM phone, then it needs to be unlocked.
FreedomPop uses both the AT&T network and the Sprint network to provide cellular service. FreedomPop will give you a SIM card for your unlocked GSM phone that will use AT&T LTE service.
If you have a CDMA phone, that’s more complicated. FreedomPop uses Sprint's CDMA network to provide service to its CDMA phone customers. These phones do not use SIM Cards. Sprint pairs a SIM to the phone's MEID numbers.
Therefore, if you have a “Sprint Locked” phone, that should work. Some newer CDMA phones are a hybrid — they are “Sprint Unlocked” but also contain a SIM tray to use for other networks. Those would work too.