The Samsung S6 should work here - it is a GSM phone.
Double check the modiel is "Compatible with GSM networks such as AT&T, T-Mobile and certain regional carriers in the U.S. Compatible bands include: LTE 2, 4, 12; LTE Roaming 1, 3, 5, 7, 17, 20; UMTS 1, 2, 4, 5; GSM Quad Band 800, 900, 1800, 1900."
As mentioned on the Samsung site...
