MTN account debt collectors a ****?

I had an MTN account that was outstanding, I paid in full at MTN. The amount I paid was as per statement received. Now I’m getting numerous calls about my outstanding account. Shapiro Shailk consultants want me to pay the money into a separate account, not directly linked to MTN. Two week ago I renewed my contract and this company keeps on calling me saying my account is outstanding. How is this even possible?? All MTN stores tell me my account is up to date. I went to the extent of even checking my credit report, there is no sign of default.