watch the plans at wal mart, you can get straight talk in either cdma or gsm versions. the blue map on the front is ATT and the red map indicates its Verizon.. I have the ATT go phone plan and have the samsung s-6 as does the wife..i havend had a minutes problem with my ATT or her straight talk on ATT towers.. 40 bucs a month unlimited everything,,after 5 gig it slows down but i have never hit that much yet.
I have no clue about cdma or gsm. It's 4G LTE on Verizon towers. Never had a problem