Unless you have permission from your carrier, you can't use your phone on another network, including overseas.
From PC Mag…
Starting Saturday, it will be illegal for you to unlock your phone without permission from your wireless carrier. The rule change dates back to an October decision from the Library of Congress’s Copyright Office, which gave consumers a 90-day window to unlock their phones without carrier permission. That 90-day period, however, ends [Jan. 26].
That means users can’t transfer a phone to another network, including to save on phone costs when traveling abroad, and it also probably hurts resale values. It could be up to a $2,500 fine for breaking the rule. The alternative – and the justification for the change – is that many carriers now offer (more expensive) unlockable phones or give permission if you’re not on a subsidized contract.