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I am by no means an early adopter. I tend to things and use them until they are no longer salvageable. I'm also a US expat living in South Korea.

When I first moved to South Korea three years ago, I had an old US phone that couldn't even handle 3G, so I happily upgraded. Now the same phone isn't compatible with the US 4G networks. I make frequent trips back to the US and need a phone that will pick up 4G in both countries.

I'm making another trip next month and I'm planning to buy an unlocked T-mobile 4G-capable phone for use in the US. Then just swapping the SIM cards when I'm back in South Korea and continue to use it as my main phone here.

Has anyone tried this? What should I watch for?

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I have done this for several years without any problems. There are a couple things that you need to do:

  1. Research your Korean provider. (Let's take SKT as an example.)
  2. Find out what frequencies / technologies they use. This info is all on the company's page on Wikipedia.
  3. Make sure that you choose a phone that supports those frequencies / bands. You can find this information all over the internet.

The only caution is that there are several "channels" for LTE. Make sure that you're not just picking LTE vs CDMA vs GSM, but the actual channels or frequencies within the technology.

The only phones in the last few years that seemed to have major problems were the iPhone 5, which did not support LTE in Korea at all, and the iPhone 4, where the Verizon and Sprint models did not support WCDMA (but only CDMA). All Samsung phones didn't seem to have an issue, nor did LG, Google Nexus family phones, Motorola phones, or most Windows phones.

Your mileage may vary, but I have had great experiences (all except for the iPhone 5).

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T-Mobile uses GSM, but South Korea uses CDMA. You might (?) be able to use the 4G LTE part, but this seems like a poor idea, because any areas in South Korea which don't have 4G LTE coverage will have zero coverage for your US-bought phone.

I think you're better off buying a cheap smartphone that works properly in the US.

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