Hi Guys
I don't knoew if this will help but this is my story of setting up 6 brand new iphone this last week.
We upgraded all our Blackberries from T-Mobile and Ornage to new iPhone 5's with EE - all carriers (or ex carriers) in the UK
With the phone came new Nano Sims which we pu in and setup the iphones with great success.....until we started sending and recieving texst messages - SMS work ok but iMessages looked like they came from totally different numbers. Also the guys with the phone started getting Facetimes from a nice young lady. She turned out to be a girlfriend of one of the lads with a new iphone.
What happened is that when we got the new nano SIMS they came with a brand new number, it then takes 30 minutes for the old numbers to be ported back over - however we had setup the iphones in this waiting period and so the brand new throwaway numbers were embedded into imessage and Facetime.
So today I switched off facetime and imessage etc to stop confusion and Googled for a way to change the embeeded number and stumbled across this page. When trying one of the suggestions it meant tuning backon imessage and to my suprise it was now using the phones correct number - hurrah
So I think if you have to change the number in imessage or facetime just switch off imessage under Setting->Messages and turn it back on.
Hope this works for you all.
MarkyB