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The most useful answer from a comment by @Fattie
In answer to your exact question, generally once you "pay out" the lock period, yes, it's your phone to do what you like with. However there is a HUGE fuss at the end to unlock it. I would truly urge you to simply buy a totally unlocked phone. Buy one now before you leave. Then you can buy any sim you like -- swisscom, Lyca, whatever.