Fiber line speed

I had requested a downgrade on my Fiber speed however I was advised by Operations that SADV had been taken over by Vumatel and they don't have 20 20 Fiber speed line on Vumatel but could offer me the 50 50 line speed at R698, which I accepted. On the 01 May I was downgraded to a 20 20 line and charged R798. I have now been told that I am on the Vumatel 50 50 line but I am only getting 18mbps and I have called the call centre numerous times to have my account and line speed rectified as promised however no one even returns my calls. Worst service ever.