CONSTANT HARRASSMENT on FNB CC repairments

We came to an agreement to pay off credit card from March. 1st payment due 30th March, from 25th of the month received daily sms's and phone calls asking when the payment would be made.... seriously it is in writing 30 March. We make payment and we still get a call to say when is payment being made. I then get a call to say thank you for payment. I ask very nicely for the calls and sms's to stop as they are just costs going onto the account and are totally unnecessary. I eventually get an Agent who agrees and says she will make a note to just communicate by email. Last week the calls start up again from Bloem, Durban and today PE. 2-3 calls a day and when I answer them I either get the phone slammed down on me or I am told I am not allowed to discuss the matter even though there is a power of attorney on the file. I have now told the last Agent I will take legal action if this constant harrassment is not stopped. I called the Head office to speak to a Manager, was put onto a hold line with music for 2 mins and then was cut off.....