Problem in calling MTN numbers from U.K.

I have been in the U.K. since 2007 and have had the same cell number for that period. I have called my family and friends who are on the MTN network for all that time with no problems. I came to South Africa on 12th December, 2014 and was able to call MTN numbers up until I left. I returned to the U.K. on 17th January, 2015 and tried to call family on the MTN network when I returned but I was unable to call anybody on MTN. All I got when I call MTN is a long beep sound and then the call cuts off. I can call my family and friends who are on Vodacom or Telkom, but not MTN. I went to my service provider, O2, in the U.K. and they tried on my phone and another phone, and had the same problem. They tried on their landline and were able to connect to MTN. They spoke with their technical staff, who told me that the problem was on the MTN network, I contacted MTN about the problem on 1st February, 2015, to ask them to resolve the problem. They came back to me with unsatisfactory explanations. I contacted them again and eventually they said that a technical person would call me. They called on 24th Feb, but I missed the call. I have tried several times to get a response without success.