It is not a mystery number. It is a hidden underlying carrier number used to route incoming calls over the partner carrier cell network. All R.W phones have this, even you old Moto X. You need not concern yourself with it nor should it be used as it can sometimes change.
Number spoofing is when the outgoing caller ID info is changed so that when someone else calls another person it shows that number.
Are you using the normal built in dialer app and not a 3rd party dialer app like Google Voice?
Does your R.W number show up in the R.W. app and is it the same one in the phones System settings, About Phone, My Number screen?