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I have a smartphone bought in the US through a US carrier, not rooted or otherwise modified. I understand that some phones will work overseas, some won't, and some can if I make modifications to the phone. How do I figure out if any given phone will work in any given country?

I imagine that the factors include hardware (specific phone, network technology), who my carrier is, and exactly where I'm going. I'm not asking about costs; I understand that overseas use might be expensive and I would have to check with my carrier and my specific plan. I'm asking how to know, before going, if I'll be able to make calls and use data at all (other than through local wireless networks).

I'm trying to ask this question generally, rather than about one particular phone that will make the question obsolete in a year.

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IMPO I don't think there can be a general answer to such a question. Even for posterity. The best way is to call your provider a week before you leave and confirm about the specifics wrt the destination country(ies). –  happybuddha Dec 15 '13 at 15:34
    
Why would asking about a particular phone make the question obsolete in a year? I know some smartphones are bad, but do they self-destruct after one year? –  gerrit Dec 16 '13 at 11:54
    
@gerrit ok, maybe not in a year exactly, but I thought we were generally aiming for questions with broader application than "will phone X work in country Y?". But I don't have a lot of experience on this particular site (others on SE yes, but not here), so if y'all think I should ask that I'm happy to. –  Monica Cellio Dec 16 '13 at 13:16

2 Answers 2

up vote 10 down vote accepted

Cell phones run on two different types of networks: GSM and CDMA. These network types themselves are subdivided into various bands (e.g. GSM 850 MHz). Most recent phones will support a wide range of bands within one network type. GSM is predominant in most countries except the US, which uses both.

To verify that your current phone will work in another country, you first need to find out which bands your phone works on. This will be listed on the technical specifications, which you may be able to find online (if you can't, try calling your carrier). You'll then need to check which bands are used by carriers in the country you're going to. Wikipedia is good for this—for example, here's the list for Europe.

Data basically works the same way, although it is a bit more complicated. Basically, there are a number of different network technologies: HSDPA, HSUPA, HSPA, HSPA+, LTE (I'm not sure how complete this list is). The best way to do this is to check Wikipedia and work backwards (if their network supports, say, 1800 MHz LTE, you'll want to check if your phone does).

In addition, if your phone has not been unlocked, you will be forced to keep using your current SIM. This seems to be what you're planning to do, but I just want to emphasize that switching to a local SIM will not work, so if your carrier is willing to unlock your phone, do get that done.

The other thing to make sure to do is check with your carrier is that you do have international roaming enabled. If not, your phone basically won't work outside of the US, and it's a lot easier to get that enabled before you travel instead of trying to dial your carrier from abroad.

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Just one thing. Most mobile operators have partners in almost all countries. You dont need to have your phone unlocked if you are going to be using your operator's partner's network in that country. This also changes if you haven't spent X amount of time with the operator in the US. (T-Mobile will not unlock your phone if you haven't been using that phone on their network for atleast a week.) –  happybuddha Dec 15 '13 at 15:32
    
Indonesia also uses CMDA along with GSM just like the US. –  MeNoTalk Dec 15 '13 at 16:43
    
+1. Good advice mainly. Unlock restrictions can be tighter than you may hope. My wife has had a Vodafone phone from NZ for many years but Vodafone Australia (next country across Tasman sea) said they were "unable" to unlock it for use on the Vodafone Australia network. (Which is typical of the sort of "service" I've come to expect of Vodafone). I have a multi-band unlocked GSM Android phone but using a Telstra sim it would not work on voice in some outback areas that had Telstra service even when emergency calls worked (ie limitation was sometimes operator not technology) & data was worse. –  Russell McMahon Dec 15 '13 at 17:40

I understand you intend to use your phone with the original network provisioning of your operator (e.g. the original SIM card); if so, the question has two parts to it:

  • is the phone physically capable of connecting to a foreign network?

    • is the foreign network technology (e.g. GSM, UMTS, LTE, or sometimes cdmaOne/CDMA2000) supported by your phone?
    • are the frequencies for a given technology match?
  • is an agreement in place between your operator and the foreign operator, and whether it covers the technology and frequency combinations that you phone supports?

The network technology

Some technologies are backwards compatible with others, whereas others are incompatible with each other. The compatibility is denoted by a technology family of a single generation, and the frequency over which it is used.

For example, most people don't realise that phones that only support GSM are incompatible with UMTS/HSPA-only networks, which do exist.

You need to figure out the technologies that your phone supports, and jot down the bands for each technology.

This is a list of some common standards, and some substandards that are all backwards compatible within each other:

  • GSM: GRPS, EDGE.
    • Popular GSM frequencies are 850MHz, 900MHz, 1800MHz and 1900MHz.
  • UMTS: W-CDMA, HSPA, HSDPA, HSUPA, HSPA+ etc.
    • Popular UMTS frequencies are 850MHz, 900MHz, 1700/2100MHz (that's a single frequency, called AWS), 1900MHz, 2100MHz (not to be confused with 1700/2100MHz, which is a separate, and incompatible, frequency).
  • LTE.
    • It has so many slightly-different bands, that it became meaningless to use the MHz notation for each frequency, and most frequencies are referred to through some synthetic numbers from the LTE family of standards. Unlike GSM and UMTS, which support voice calls from the start, not all LTE networks may support voice calls (the responsibility may be delegated to the UMTS/GSM or the CDMA part of the network).
  • There's also cdmaOne and CDMA2000 (1xRTT, EV-DO), these are actually distinct generations, but are backwards compatible with one another; they're mostly popular only in US/Canada and Japan/Korea.
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