In the US, when I tell people about I have an unlocked (carrier-independent) phone, they’re often surprised. Where do you get one of those? Is that legal?
I bought mine on Amazon, which is, unless I’m very misinformed, perfectly legal. And once you have one, it’s relatively simple to activate a carrier-independent GSM phone with a SIM card. Unfortunately, there are only two big carriers that use SIM cards in America: AT&T and T-Mobile. (Here’s a handy list of all US wireless carriers if you’re interested.) You can buy a new SIM card directly from AT&T or T-Mobile for around $5 or just get one at Amazon or another store.
The part where things get complicated is ensuring that your phone can communicate on the right bands or frequencies. It used to be OK to just buy a global “quad-band” phone, but now you need to worry about high-speed data too (assuming you’re getting a phone capable of transferring data at 3G or 4G, and seriously, who isn’t?).
Here are the frequencies you need to work with AT&T and T-Mobile in the US:
|Carrier||GSM bands for 2G/EDGE connection||HSPA (HSUPA/HSDPA) bands for 3G/4G connection|
|AT&T||GSM 850 and 1900 MHz||HSPA 850 and 1900 MHz|
|T-Mobile||GSM 1700 or 2100 MHz||HSPA 1700 and 2100 MHz|
I’ve heard people say they’ll settle for the slower EDGE network and use wi-fi for broadband, which is reasonable, but you should know what you’re getting into. Your phone needs to support both 850 MHz and 1900 MHz to get 3G+ speeds on AT&T in the US. I’ve heard their are exceptions depending on the part of the country you’re in, but if you want your phone to connect to 3G reliably, you should make sure to get a phone that works across these bands. Likewise you need both 1700 and 2100 MHz support for faster data speeds on T-Mobile US.
If you really want to try out a phone that only supports one of the bands your carrier uses, check out this AT&T US coverage map by frequency to check your area. I can’t find a map for T-Mobile by frequency, but the map on their site is nice. I don’t suggest roughing it with shabby coverage, but it’s your party.
Disclaimer: I have experience with this, but I’m not an engineer, so if I have my facts wrong, please let me know :)