Do NOT trust random unlock code offers. If the phone is locked then go check out the device on phone tech sites to find out if its even unlockable.
If you need to have someone unlock it then go someplace locally to make SURE that they actually do what you pay them for. (and if they brick it then its on them)
Also, you might want to check what bands its on. Most Tmo handsets are on AWS/1700 spectrum which is not going to do you any good whatsoever on ATT. If it has 850/1900 then you should be fine. Tmo is actively deploying 1900 but older devices may not have it.
Even if its 1900/1700 there are markets where AT&T only use 850.
Tmo is bad about giving full specs on their devices, mainly because their network is fairly homogenous. Tmo's original spectrum was heavily on PCS band (its where all their GSM is) and they bought a huge slice of AWS/2100 spectrum. They have carved out a piece of PCS for mainly iPhone migrators but they don't have to provide that spectrum in their handsets.
Aha, found the specs... UMTS is only on the AWS band so it will only work GSM on ATT -- ATT is sunsetting GSM by 2014:
WCDMA / HSPA 7.2 (3G / 4G)
1700 AWS / band 4 (IV) Americas
850 Cellular Americas, Oceania, Brazil, Israel
900 Europe, Asia, Africa
1800 DCS Europe, Asia, Africa
1900 PCS Americas