unlocked my iPhone 4 from O2 recently using their service and shall be giving phone to my mum. But before give it to her want to set phone back to original factory default settings will this re-lock phone or will it remain unlocked

I unlocked my iPhone 4 from O2 recently using their service and shall be giving phone to my mum. But before I give it to her i want to set the phone back to the original factory default settings, will this re-lock the phone or will it remain unlocked?