At this point, there are no true VoIP clients for BBs. They are are all call-back services, which use your voice plan. May offer cheaper rates while on your home network, but offer no apparent savings if you're roaming.
As mentioned, AT&T doesn't do UMA, only T-Mob does, and then only on T-Mob BB. Until someone develops a true VOIP client for wifi-enabled BBs, we're all out of luck