Paying for a contract, but supplier do not perform

I have a open contract with MTN, since my free SMS bundel have been used up I am unable to sent any sms. I have phoned the call centre now several times, every time you have to wait for hours, MTN are experiencing high cal volumes, on a week day and over the weekend you will get the same message. I have spoken to two consultants, the first said he must reset my sim, I must restart my phone and it will al be fine. Unfortunetaly I can still not sent a SMS. I phoned again, the next consultant did not know why it is happening and just want to foreard me to other departments. Needles to say I can still not sent a SMS. But I still need to pay my full subscription for services I am unable to use. Can YOU MTN please reduse my contract fee for services that YOU are unable to deliver????