Unauthorized deduction and No assistance

I was charged R604 for a service fee after I upgraded my contract. I have two contracts with MTN and mtn can't account for that amount instead they are taking me from pillar to post. I called MTN they sent me to summer mobile I called summer mobile they referred me back to MTN no one is assisting all this time I'm using my airtime and hold for hours only to get zero assistance. I was not even informed about this service fee beforehand.