Exceptionally bad service

For the past 5 months I have not been able to make or receive calls at my home in Plettenberg Bay even thought I have 3 bars showing on my phone. This problem has been reported by myself and my neighbours with absolutely no success? As I cannot make calls I only use +/- 20 minutes of my 300 minutes I receive monthly. I decided to change from contract to prepaid. Visited the store in Plettenberg Bay and was told to contact 135 as they could not assist. I dialed 135 spoke to a lady told her what I wanted to do and was transferred to retentions@mtn after 35 minutes of waiting my phone switch off as it got hot in the sun. Remember I cannot call from home so had to sit in my car.

As I had no luck phoning I sent retentions@mtn an email. It was delivered but I received no response so I sent another mail and still no response.

Does anyone know how I can change from contract to prepaid without having to dial 135.