No manners call centre agents at collections

I have made payment arrangements with African Bank in terms of what I can afford. Every month before payday I get a call from a VERY RUDE agent, they have no manners, do not listen to what you tell them, and have even had one of them shout at me. This is unacceptable. I do take ownership in terms of my debt but all credit institutions are obliged to accept ANY payment arrangement. I have been told by their agents on numerous occasions that this is unacceptable! I am tired of being harassed by them. They are now resorting to calling me at my workplace, I now hang up the phone when they call. I send them an email every month with my proof of payment and still they call me every month before payday and HARASS me and are rude to me. Something needs to be done about this. It is shocking! I have dealt with other call centre agents in relation to payment arrangements and have never had them be rude. What is the solution to this?