As Glenn Fleishman, wireless networking expert, points out in his latest blog, "One of the dirtiest barely secrets of the modern mobile cell world is that AT&T doesn't really have national 2G coverage, much less 3G. AT&T leans on T-Mobile for a large number of areas it never spent to cover. This stems from an agreement years ago when AT&T Wireless consolidated on GSM service, and T-Mobile was building out its initial GSM service. In 2004, the companies dissolved a cooperative agreement (when Cingular bought what was then AT&T Wireless), but roaming never disappeared."
Fleishman continued, "This lack of coverage is why AT&T didn't offer feature phone or smartphone service in large parts of the country outside urban areas. … AT&T also gets the depth of T-Mobile's spectrum portfolio in dense markets where AT&T clearly lacks the ability to deliver service to the level needed, such as New York City's boroughs and San Francisco. It won't be trivial to integrate the networks, but many carriers co-locate equipment with tower and building owners. And if they maintain the current deal and roaming is no longer a for-fee arrangement, AT&T can instantly get the benefit."
The two have also long been working together on their Wi-Fi offering. Odds are that if you were using T-Mobile Wi-Fi, your signal was actually being picked up by AT&T access points (AP)s and being routed over AT&T's share of the Internet.
Eventually, AT&T and T-Mobile, which is already being called T-Mo, will move to LTE, but I don't see this happening anytime soon. In the meantime, speed-happy mobile users will remain stuck in the LTE+ lane. Of course, at this point, there's more hype than there is speed in everyone's 4G claims.
The current joke going around runs: "AT&T is getting married to T-Mobile. There will be no reception afterwards" It's funny, but technically speaking it won't be true. As ABI Research senior analyst Mark Beccue says, "It is most likely a good thing for current AT&T mobile customers as network coverage and quality should improve with the addition of T-Mobile's cell sites." Technically and as far as infrastructure goes, the two should do well together. After all, they already are. If the deal goes through Sprint should be very worried.