PHONE RINGS ONCE THEN GOES TO VOICEMAIL MTN

I have been struggling with this problem for three weeks now, going back and forth with MTN to try and fix it. The problem: My phone rings once (Iphone 5s) and then goes straight to voicemail. I am loosing important calls on a daily basis, and my service supplier is not really interested to help me fix it. <br> <br> It seems that once the contract is signed their job is done? <br> <br> Please help MTN, or perhaps I should try and alternative buy out supplier, since it seems very view people are happy with the service, reception etc.