I have been having this problem ever since i buy new iPhone 4S in Australia Melbourne since first came out and fixed today.
Basically my problem was i can send iMessage but no SMS to standard phones. And my iMessage and facetime wouldn't activate at all even though i have the right plan with lots of credit.
After extensive hours of backups and restores i have fixed and is only small issue.
Happy to help you all.
Here is the problem.
1) First check you have enough credit or BALANCE in your phones bill account. If you do and have valid SMS function enabled from your carrier then proceed to next step, Please call your phone carrier make sure you have credit and sms is enabled.
2) Ensure you are still logged in to your apple id in message and facetime however Turn off iMessage and Facetime (settings-> imessage & settings -> facetime)
3) Please type in phone -> keypad *#5005*7672# and then press CALL
This will show you your SMS service center number.
Now please note if it does provide you with a number write it down, now Phone you phone carrier and ask them if this is the correct SMS service center number for your Contract/Prepaid/Postpaid plan. verify with them. If it isn't get the new number. You can also look on GOOGLE for the right service number just search it.
If there is no number/address showing then this is also the problem which was my same problem
So i found my SMS service center number which is for Virgin in Australia number was +61411990010
Now when you get correct the service center number you type it together in keypad and press call:
So What i typed is
*5005*7672*+61411990010#
You just must replaced the +61411990010 with your carriers service center number and your done.
Once the number is set you now goto settings iMessage and Facetime (settings-> imessage & settings -> facetime) turn them on as you desire! All works. Provided that your service center number was set correctly.
I have been on the phone to APPLE customer support they could not even figure this out i figured it out after so long.
Good luck i hope this works for you all!