Same thing here. Message settings show the alien number but it can't be manually changed.
We're assuming when porting your number from one carrier to another, if syncing with the temp number active on your SIM card, it may just save that number into your Apple ID.
Might be worth logging into your account on iTunes and checking what details it has about you and then re-syncing
Checked out iTunes, and the number that iMessages were being sent from was not on the account. However, we switched off iMessages on the iPhone, logged into iTunes and changed the contact number from a landline number (that was there) to the correct mobile number, and then switched iMessages back on. It is all working as expected now.