CANNOT SEND SMSes, FOR MONTHS NOW ???

I have been unable to send SMSes for months now. Tried the MTN call centre on numerous occasions and do everything they tell me to do, still no luck, went to their outlets many times including today, switching off my phone for hours as per their advice after they did their thing, no difference. I have dialed #330*0000# and when I send a test sms, i get "Failed to send" Please can someone help as this is frustrating, I'm pulling my hair here ??????