network unlock

See this link and be advised that only the carrier can unlock the phone and they will only do that if your phone qualifies to be unlocked --> How to unlock your iPhone for use with a different carrier - Apple Support

Note: They may require a lump sum payment to unlock the phone if you don't plan to use their services.

You need to contact the cell service provider it is locked with and see what their instructions are to unlock it. Did you buy this phone at an Apple Store and specify unlocked?