Worst experience with Nimble and their call centre agents

The worst service one can ever receive, their call centre agents are rude and dont even listen to you talking to them,

there was a coprprate account that was errorneously registered under my name by MTN, the account weas subsequently handed over to Nimble for collection. MTN has resolved the matter with our office, they sent a letter to that efffect and the same letter was sent to nimble more than 5 times, howver i still get calls from call centre agents. The call cntre agents dont even listen to you explaning or providing them a little background