Help t-mobile cliq xt unlocked now on AT&T will not update or install new apps

[tcarlson]

My unlocked t-mobile Cliq XT has worked well with AT&T, I get the +11-216-116-11 error messages on restart which is no big deal. What is an issue though, is that the phone will NOT install any new Apps or accept updates for apps like Facebook. It always gives an "Download was unsuccessful, please try again". It gives the same error from with apps off the internet and the "Android Market" link on the phone. Is this because the phone is still linked to T-mobile and is trying to update through t-mobile network? Do I need to erase all the programming and reinstall Android to free the phone from T-mobile? I really do not want to wipe out the whole phone, just want to update and add some apps to an otherwise great phone. Thank you for an advice.