Uncalled for calls, unprofessional and ******** to information

I am so unhappy how this debt collector's company work! My exact account was handed over to them, I have been battling financial but I managed to come to terms to make payments arrangement with them and which I am paying for the past two months. My problem now is they keep on calling me not knowing why? I have instructed them not to call me unless the is no payment made as per arrangement for the card payment for 15 months. The is a lady who's name is Mathapelo apparently she's the one who keeps on calling, already spoken to several consultant telling them to capture clear notes that I don't want a phone calls because I am paying the account. Can I get a valid reason for the calls, not a thank you payment because I know that every call they make I get charged for the call.