If everything goes according to AT&T's grand plan, T-Mobile customers will be offered two choices in about a year. The first is go get sucked into AT&T with its dropped calls and legendary bad service, or they can find another carrier. Fortunately for T-Mobile customers who don't want to be part of AT&T, there are options-probably more options than you suspected.
To find out just what those options are, I first looked for a list of every GSM carrier in the U.S. The closest I could find was a list on Wikipedia of U.S. wireless carriers. Using that list, I called every GSM carrier in the U.S., except for a few in Alaska (because of time-zone issues), and asked them if a T-Mobile customer would be welcome there. I didn't reach all of them, but of those I talked to, the answer was a resounding "yes." The GSM carriers out there that aren't T-Mobile or AT&T would be delighted to have your business.
But they're not the only ones. Verizon Wireless and Sprint are both eager to welcome T-Mobile customers, and both are willing to solve that thorny problem of international roaming. Verizon Wireless offers 14 phones that work with both CDMA (Code Division Multiple Access) and GSM, and a Verizon Wireless spokesperson told me that the company is happy to unlock the SIM so that you can use foreign SIM (Subscriber Identity Module) cards while you travel, provided your account is up-to-date. Of the 14 Verizon Wireless phones, four are "certified pre-owned," which is a good way to get a refurbished international smart phone at a really cheap price. To find them, visit the Verizon Wireless site, shop for phones and click on "Global Ready."
Sprint also offers four devices that can be used internationally. To find those, go to Sprint's site and shop for phones, then click on the "international" check box. Sprint will also let you use foreign SIM cards in their phones. In any case, choosing either Verizon Wireless or Sprint will probably give you better coverage than you had with T-Mobile, and it will avoid the problems of dealing with AT&T. Of the two services, my experience in general has been that Verizon Wireless has somewhat better coverage, and that the 3G and 4G service is better.
In addition, there are other GSM companies that operate nationally. One of the companies I talked to was Indigo Wireless, which is a regional carrier serving parts of Pennsylvania and Nebraska. But it accepts customers located anywhere. Then there is Fuzion Mobile, which is a smaller, but national GSM carrier. Fuzion Mobile is in the process of opening hundreds of stores throughout the U.S. Both these carriers offer plans that are less expensive than anything AT&T offers, and both provide their own customer service centers.