«T-Mobile and AT&T both utilize network technology based on GSM and UMTS standards, which allows for this sharing of voice and data traffic.»
I wonder whether UMTS voice and data roaming were actually enabled. Not sure who'd have better coverage and be outage-less on the 2G GSM side, but with 3G UMTS there comes a handset problem: probably none of AT&T handsets support AWS, whereas some, but not all, T-Mo ones support at&t 3G frequencies.
As such, has UMTS roaming actually been enabled at all? Cooperation is good either way, though.