How to unlock an old iPhone 6?

My wife, Belinda, is using a new iPhone and a friend wants to buy her old iPhone 6. She's using the present phone so we don't want to interrupt that service but we do want to wipe the old one. The old phone says, "no service" so when I go to settings/General/Reset/Erase All Content and Settings nothing happens when I press it. Can you help? Thanks.