They are a bunch of bullies who claim to be collecting on behalf of Telkom

They are a bunch of bullies who claim to be collecting on behalf of Telkom. Although I have notified them that I have already made arrangements and started making payments to another debt collector who is collecting on behalf of Telkom they insist on verifying my personal details and harass me daily. The person who called me today claims that Telkom has a right to hand over my account to as many debt collectors as they wish until they have successfully collected the outstanding amount. This does not make sense and how do I get them off my back? Should Telkom not inform their clients that the account has been handed over and to whom so we as customers are not ****med into paying multiple people for the same account!!