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I've got some questions about how GSM works. I'm thinking of purchasing a GSM phone in the next few years when I begin to travel overseas.

1. How likely is it that if I take an unlocked phone to a carrier, I can use it? (i.e, do a lot of carriers let you use unlocked phones overseas)

2. Is HSPA+ the big thing in Europe, or is another form of 4G going to be implemented?

3. Are carriers all country specific?

4. How exactly does coverage work with GSM, do I go to a carrier and they give me my card and I only get coverage within the country, then I need a whole different carrier if I leave the country?

5. Is HSPA+ widely available in Europe?

6. Is HSPA+ expensive?

7. Is t-mobile good for international coverage, and is it expensive?

I can't find any info on this, sorry for asking so many questions.