What phone it is really isn't the issue. What network it was on originally is. AT&T is a GSM network, so the network you had the phone on originally, when you got it would have to also be GSM. If the phone, however, was on a CDMA network, like say Sprint, then you can't take it over to AT&T.
If all the info I have located is correct, C Spire is indeed CDMA, so the phone would not work with AT&T.
Porting has nothing to do with what network you are using. It has to do with a phone number. You can 'port' a phone number from one carrier/service provider to another, that is if the phone will work on that carrier's/service provider's network. If yours is CDMA, then it won't work on AT&T's or Cricket's GSM network.
You can find a list of...