Currently Being ModeratedSep 3, 2013 6:31 AM (in response to Rcbee)
Enable SMS under Setttings - Messages
Currently Being ModeratedSep 3, 2013 6:33 AM (in response to Rcbee)
Settings -> Messages
Do you have "Send as SMS" ON?
If not, then you need to enable that - green messages are being sent/received via SMS text messaging through your phone's service provider, not by Apples iMessage service.
If you have send as SMS enabled, then try contacting your carrier and make sure that SMS texting is set up properly on your account at their end.