1. Go to settings, Icloud and delete account. Put your Apple id to delete but keep content in your Iphone.
2. Go to settings, itunes and appstore and click on your apple id and select sign out
3. Then go to imessage, send and receive, click on your apple id and select sign out
4. Do the same with facetime. At this point imessage and facetime is turned off and e-mail info and apple id are not set for appstore,icloud,imessage and facetime.
5. Power off your phone and wait for 10 minutes
6. Turn on your phone. Go to imessage and turn it on and wait for a couple of minutes to activate. DO NOT use your apple id for imessage because that's the reason the # does not register. After your # is ok then you can add another e-mail and sign in your icloud and appstore account.
Greetings from Greece