go the the settings all and tap on "Messages". Under there you will see an item that says "Send & Receive" on the left and something like 2 addresses (number might be different) on the right. Tap that. Now you can add/remove email addresses (AppleIDs) that are associated with imessage and tap to deactivate the phone number i believe. Not sure how to add a different phone number other than the observation that mine is my iohone number and in my contacts as my ID card.
I made a new AppleID just for use with my wifes iPad since we use a shared ID for purchases. I put her new one in that location and now we can message bac and forth without seeing everything twice.
Using FaceTime http://support.apple.com/kb/ht4319
Troubleshooting FaceTime http://support.apple.com/kb/TS3367
The Complete Guide to FaceTime + iMessage: Setup, Use, and Troubleshooting
Troubleshooting FaceTime and iMessage activation
Using FaceTime and iMessage behind a firewall
iOS: About Messages
Set up iMessage
iOS 6 and OS X Mountain Lion: Link your phone number and Apple ID for use with FaceTime and iMessage
How to Set Up & Use iMessage on iPhone, iPad, & iPod touch with iOS
Troubleshooting iMessage Issues: Some Useful Tips You Should Try
Setting Up Multiple iOS Devices for iMessage and Facetime
FaceTime and iMessage not accepting Apple ID password
Unable to use FaceTime and iMessage with my apple ID
How to Block Someone on FaceTime
My Facetime Doesn't Ring
To send messages to non-Apple devices, check out the TextFree app https://itunes.apple.com/us/app/text-free-textfree-sms-real/id399355755?mt=8
How to Send SMS from iPad
You can check the status of the FaceTime/iMessage servers at this link.
iOS: About Messages
This seems to confirm my thesis that to associate an iPad or iPod Touch with a phone number, I need to be running IOS 6 on the iPhone. I will test this out when I get home.
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.