Smart Talk uses all 4 of the Big 4 carriers - including AT&T and Verizon, so if the SIM physically fits in the Grand Prime, just swap it to the Grand Prime. Smart Talk may ask if you changed phones (they usually aren't smart enough to) but "yes" is enough of an answer. Just make sure it's a GSM phone - otherwise you'll have to pay ST for a CDMA SIM (it's the same thing, but it won't work in a GSM phone - other providers don't have the option of charging you for a SIM that most of them provide for free - ST does.)