In a word...no. While both carriers are GSM and on the same frequency, the phone are locked to a carrier. You can do an internet search for GSM phone unlock codes. The unlock code will unlock the carrier code so it can be used on any GSM service provided you have an active sim card for that carrier.
i am about to purchase a phone but it was used on a t-mobile service. can i use my AT&T sim card in the t-mobile phone? any help would be greatly appreciated