Oh, Apple. I loaned my spare iPhone to my brother for a few months when he broke his. Got it back yesterday. I've wiped the phone (factory reset), set up a new Apple ID for it, and just installed iOS 11…and it still shows his phone number.
Now, I don't have a SIM in it at the moment, which might be why. But I do wonder why it's showing any number at all if there's no SIM…and why that information survived the factory reset.