Ask your cellular provider to unlock the phone. They are the only ones who can.
If you are referring to a passcode:
Locked Out, Forgot Lock or Restrictions Passcode, or Need to Restore Your Device
8. RecBoot: Easy Way to Put iPhone into Recovery Mode - if all else fails.
Forgotten Restrictions Passcode Help
If this method does not work, then you will need to fully Restore your
device as New
Also, see iTunes- Restoring iOS software.