UNABLE TO MAKE OUTGOING CALLS

I recently purchased a contract through MTN, and one month into the contract, out of the blue I am unable to make outgoing calls, when I do the call cuts immediately, I am however able to receive calls. I've tried numerous things from resetting network settings, shutting the phone off and restarting the phone. I've checked the call barring and it is disabled. I can't get in-touch with them as there is no answer on their side. I have now resorted to emailing them and still await a response. This is absolutely frustrating.