Waiting for activation

Start here, If you get an error when trying to activate iMessage or FaceTime - Apple Support

Did you get this used iPhone directly from Apple or a third-party source? If the latter, it appears that the original owner did not properly prepare this phone for resale. If you are able to contact them, provide them with the following article so that they can remove their Apple ID from this phone:

• What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch - Apple Support

Sorry, I mis-read your post. It looks like ChrisJ4203 didn't and is giving you the proper solution. Thanks @ChrisJ4203!

You're welcome, but we both may be somewhat right. Reading the OP's post again, would make me wonder why there may be a number already there that they are trying to change.