If you have old iPhone friends who used iMessage and you are trying to message with them then you MUST turn off imessage from your iphone number otherwise it won't work.
Folks the answer is not what you would think...I have had this problem and SCOURED 30-40 fora with no luck. The answer is counter-intuitive - Turn OFF group text as this feature seems to be dependent on your Mobile provider. It worked, immediately, for me.
John.