I inserted sim card but no service is showing

Contact your phone company (carrier).

If you see No Service, Searching, or SOS on your iPhone or iPad - Apple Support

Note that a SIM won't work if your iPhone is locked to a different carrier. In that case, you would have to ask the carrier who holds the lock to release it. Which they might not choose to do.

How to unlock your iPhone for use with a different carrier - Apple Support