can't use the "free" sms that are part of my contract package

I had a data contract with the company. Upon expiry, I upgraded to a voice contract (where I got voice, data and sms). I however was unable to use the sms as everytime i tried sending it failed. I called MTN about this problem,more than once but they failed to help me. I eventually gave up. So through out the contract i only managed to use the data and minutes but not sms's. I have upgraded the same number, and again the same problem is happening. So, I am not getting value for money as the sms's I can't send yet I am paying the full amount every month. If MTN can't help me, who else will? Why am i still paying the full contract price every month yet am not getting the complete set of services?