How to remove a number from an iPhone 5c?

I got a iPhone 5c from a coworker and want to use it without cellular service, like an iPod. Problem is, every time I connect to Wi-Fi, I get her old messages, because the phone is still connected to her number. I tried to remove the number in the settings but the number is in gray and I can't un-click it.

Go into the settings and restore it to factory settings.

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