If you’re on AT&T or T-Mobile, your phone uses GSM to connect to the network. (Verizon and Sprint use CDMA).
GSM phones need SIMs from your network (an AT&T SIM card, or a T-Mobile SIM card) so that it knows who (as in what phone number, what customer) is trying to use their network. Sprint/Verizon’s CDMA does not require any SIM, your phone number/identity is tied to the phone itself.
edit: for your phone to work overseas you need to ensure your phone is a “world phone,” that is, it is quad-band in frequency. For example when I look up my Nexus One on phonescoop, under “Modes” it lists GSM 850 / 900 / 1800 / 1900. However if we look at another cheaper phone, the Sony Ericsson Z300, under “Modes” it only lists 850/1900 (frequencies usually used in the U.S.). You can use phonescoop.com to check your phone’s specifications. By the way you should understand now why “quad band” refers to world-capable phones, as these phones support 4 frequency bands (850, 900, 1800, 1900).
GSM = Flexibility (in theory)
GSM theoretically offers flexibility with phones: if you want to stay with your network but get a new phone just move your SIM card, OR if you are in a different country and want to use local phone companies (because they may be cheaper, as in India) you can replace your AT&T/T-Mobile SIM with a local company’s SIM. Often it’s not that simple because phones in the U.S. purchased on contract are SIM-locked so you can’t drop in another SIM so easily. By the way if you’re on a CDMA network, let’s say on Verizon, and you get a new CDMA phone, you need to tell Verizon and they have to reroute all calls centrally so for some time you may be missing phone calls as moving your number to the new phone can take some time (a few hours).
SIM-Unlocked Phones FTW
This is why I recommend purchasing global (quad-band) GSM phones SIM-unlocked if you need this portability internationally. It’s extremely expensive, $500 or so, but you have the freedom to drop in any local SIM. I have already told you why you should unlock your GSM phone as soon as you can, even if it’s a phone purchased on a contract.
Unlock your phone that YOU own, for FREE!
If you currently have a quad-band GSM phone (check your model here) and you’ve been a loyal AT&T or T-Mobile customer, and (maybe) you’ve owned the phone for over a year, I highly recommend you call up your customer service department and request your phone’s SIM unlock instructions as well as unlock code.
They will probably resist, especially if you’re still on contract, but stand firm. Tell them you are planning to travel overseas to one country (pick any country overseas) and you want to use a local SIM. They’ll argue that they provide coverage in that country and that you don’t need a local SIM, but reply that this option is too expensive. They will then ask you to sign up for their international package but don’t buy into it, because usually it’s still expensive. Anyway you can tell the customer support representative that unlocking your phone does not prevent you from using their services, it just gives you more options! (yikes, competition, AT&T hates that!)
If you’re backpacking through all of Europe then it might make sense to stay with AT&T and T-Mobile because you probably won’t have time to research and purchase pre-paid SIM cards in each country. But if you’re going to be in one country for most of your trip it usually pays to use a local SIM. In India, for example, even with pre-paid SIM cards many companies don’t charge at all for incoming calls and texts which is significantly cheaper. It’s also cheaper for people in the country to call you since your phone now has a local number.
I should note that in the past (prior to a Verizon iPhone) AT&T would not help you unlock your iPhone. That may still be an issue today. I’m sure part of it is AT&T but I believe Apple is also partially to blame, as they want to ensure that people in foreign countries purchase phones from local Apple stores, and not buy out phones in the U.S. If you were able to get AT&T to help you though, please let me know in the comments!
Unlock your phone even if you won’t be travelling!
This is something you should do even if you’re not planning on traveling for a number of other reasons:
- It’s your phone, it shouldn’t be tied to any carrier
- you can now move between T-Mobile and AT&T for phone calls and Edge data (but not fast 3G, long story but they use different frequencies)
- it significantly improves the chances of your ability to resell your phone (and resale value) if you can advertise that it is SIM-unlocked as people.
Of course if AT&T is able to purchase T-Mobile then, at least among the national networks, it won’t matter if your phone is SIM-unlocked since there will not be another carrier to take your phone to!