IPhone with iOS 6 no longer receives iMessages (SMS only)

Question

I'm working with a friend who's having an odd issue with his iPhone (iOS 6).  I used to be able to use iMessage to reach him via his phone number, but now I see an error when I use that same number:  "+1 (xxx) xxx-xxx is not registered with iMessage."
We've verified that Messages is turned on and enabled on the phone.  The phone number is listed as a valid channel (alongside his iCloud e-mail) for iMessages.  We've reset the phone.  Turned Messages on and off.  Nothing seems to re-enable Messasges to his phone number. 
If I send a Message via my iPhone to his, it is always an SMS.
If I send a Message to him via his iCloud e-mail, it is always an iMessage and arrives on both iPhone and computer.
I'm really puzzled and I think we've tried everything in the support documents so I was wondering if there's something else we should tackle?

Answer 1

Hi Friend,
iMessages and Facetime are not available in some countries, so please check it first.
Be sure you do not turn on Restrictions in your device.
Try to unsign and sign in again.
For FaceTime and iMessage chosse to use your email address instead of your mobile number.
Go to Settings>>Message>>Send&Receive and set your email address.
If it is still not working, unsign your Apple ID and select your mobile number ("+" "country code" "mobile number")
Go to www.apple.com or Settings>>iCloud then access your Apple ID. There you can edit your information, placing your phone number and tap OK. then go to iMessage settings and mark your phone number to receive iMessages.
Also check in: Settings>>Message if you turned on iMessage before. Here, turn off "Show Subject Field".
Change your address for your currently location. If not works, change it for your native country.
Did you turn on iCloud as well?
Reset the Network Setting, which makes your phone to forget every networks and passwords that you connected your phone to.
to reset the Network Settings just go to Settings - General - Reset (at the bottom) - Reset Network Settings.
I remember when I reset the Network Settings, i turned off iMessages. I connected to my home Wi-Fi and surfed the web for a little while. Sent some regular SMS messages, and about 10min later, I turned on iMessages again and sent a text to a friend with an iPhone and it started working as iMessage! Baloons finally became blue.
Hope it helps