F funflea Estimable Feb 5, 2016 1 0 4,510 Feb 5, 2016 #1 I just read that an At&T 3G sim card will not work with a 2G phone. Will the 3G sim card work in a 4G phone? And, if I buy a new 4G sim card does that card required activation? Can they transfer my old number as well? Thanks !