Legit iPhone imei unlocking?

I'm kinda frustrated because my iphone is locked on a french carrier which i don't even know and i wanna update it . It is starting to work pretty bad and it could really use a total wipe and an update ( preserved BB i guess??) So ive heard about methods where you give your imei and they unlock your phone. is that legit? Does it become unlocked forever? working with any carrier without gevey/jailbreak? Thank you for your time, i could use an answer asap!:)