In the US, two cell-phone technologies are in wide use, CDMA and GSM. T-Mobile USA is a GSM carrier. In fact, among the national carriers, Verizon and Sprint use CDMA, while AT&T and T-Mobile use GSM. The takeover of T-Mobile by AT&T would reduce the number of major GSM carriers from 2 to 1.
GSM is widely used, and is the standard across Europe, where people cross borders frequently. A phone bought in Germany is technically capable of being used in France or Austria, for example. A German can visit France and his carrier will have a roaming agreement with a French carrier, and there you go, he can use his phone.
Because GSM phones normally have a removable SIM, the German has another option, to use a French carrier, and buy some prepaid minutes from that company. SIM means Subscriber Identity Module, a tiny smart card as in the picture above. The German will buy a SIM for the French carrier, and put it in place of his normal German SIM, giving him a French phone number during his visit.
He will have other freedoms, for example to move his SIM from one phone to another, according to whim or when he gets a new phone. If the German carrier subsidized his phone, it may be locked to the one carrier, but after some delay they may give him an unlock code.
Another issue is that European phones use 900 MHz and 1800 MHz bands for voice, while USA carriers use 850 MHz and 1900 MHz. However, in the USA quad band phones are available for use as “world phones.” As new technologies come into use, new incompatibilities arise. But a quad band GSM phone can at least be used for voice calls in most of the 212 countries and territories with GSM service. A chatty but frugal American will make sure that her phone is unlocked, and buy new SIMs in the countries that she visits. Travel issues are discussed on the FCC web site.
Coming back to the issue of the proposed merger, T-Mobile has a liberal and clearly stated policy for unlocking a customer’s phone after a waiting period. AT&T is reputedly less helpful. Also, with 2 competing GSM carriers, if you believe your phone is unlocked, you can test it by borrowing a friend’s SIM for the other network.
Apart from travel, there is a bit more of a free market for GSM phones, because they are used around the world. See this search . Considering the issues of travel and of shopping for phones, having 2 GSM carriers adds to the overall freedom of USA cell phone users.
UPDATE, Sunday 2011 May 8: The Washington Post has a new article on this topic , called “Travelers, phone home — it’s so SIMple,” by Andrea Sachs.