CSpire uses CDMA technology which does not use SIM cards. So, in short that means you can't use any phone but theirs on their network unlike GSM providers where you can use unlocked phones.

I am in the market for a cellphone that is compatible with CSpire service. How can I know if the phone I purchase will be compatible with CSpire service. CSpire employees discourage buying a phone from anyone except CSpire (of course!). However, their prices are out of this world!