A:Answer Phones are base on radio frequencies. If the phone has the correct internals to accept the GSM frequencies then you can switch sims. Most phones stick to CDMA (Verizon) or to GSM (AT&T) to save costs. Only the Flagships will usually have the both. but the fact that you are talking about a flip phone, you should be able to purchase one for less than $10 that will have GSM. GSM is the most popular. You will need to lookup the phone to figure this out.
Carrier 3G Frequencies 4G Bands
Verizon CDMA 800, 1900 PCS 2, 4, 5, 13*, 66
AT&T GSMA/HSPA/HSPA+ 1900 MHz, 850 MHz 2, 4, 5, 12*, 14, 17*, 29, 30, 66
T-Mobile GSM/HSPA/HSPA+ 1900 MHz, 1700/2100 MHz 2*, 4*, 5, 12*, 66, 71*
Sprint CDMA 800 MHz, 1900 MHz 25*, 26, 41
