After porting my phone number from one network to another, my iMessage and FaceTime on my iPhone were stuck using the temporary number I was given by my new network. I needed to find a way of removing that old number from my device and getting iMessage working with MY phone number.
Searching online seemed to be of no help, so I contacted Apple Support. They actually resolved my problem within a minute of the call connecting.
The simple way to update the telephone number used for iMessages on your phone is by going to Settings > Phone > My Number.
From this screen you are able to edit your phone number. As long as your SIM is assosciated with that number, you can then set it as the default contact for FaceTime and iMessage.