Wife and I each have iPhone 5's. We have an iMac. I bought an iPad mini this weekend and could not figure how to get iMessage on the mini to show my phone number as an option. My choices were my email, wife's ph number or add a new email. I was on the phone with Apple for an hour and they could not figure this out. I turned everything off and back on to no avail. I factory reset the mini to no avail. I stumbled across this post from TechAddict and FINALLY I could add my phone number! The key is to Add your phone number on Messages on your Mac as an email [1 xxx-xxx-xxxx]. It is imperative to turn off the email on the iPad as a sending/receiving source or the verification from Apple to your iPad will not work. Then I turned off Messages and FaceTime on the iPad and deleted my account from iCloud settings on iPad. I then powered off iPad just for the heck of it. When I turned it back on and signed in to iCloud, this made it work. When I turned Messages and FaceTime back on, my phone number was finally an option. Here is a summary of the steps:
1. Add your missing phone number as a new E-Mail on your Mac under preferences once Messages is open.
2. Enter the phone number on Messages>Preferences as this format 1 xxx-xxx-xxxx
3. On iPad go to Settings>Messages>Send/Receive>Delete email
4. On iPad go to Settings>Messages>Off
5. On iPad go to Settings>FaceTime>Off
6. On iPad go to Settings>iCloud>Delete Account
7. Power off iPad.
8. Power on iPad.
9. On iPad go to Settings>iCloud>Sign in
10.On iPad go to Setting>Messages>On
11.On iPad go to Settings>Messages>Send/Receive and choose the phone number that FINALLY is available and either delete or keep email address-I chose to delete the email address and use the ph # only.
12.On iPad go to Settings>FaceTime>On
I hope this saves someone the 5 hours I wasted!