Harrassed by debt collector for an account that does not belong to me

On 15 August 2017 i received a call from Thulani Nkambule, a debt collector from Hammond Pole Attorneys. He explained that I owe MTN just over R22 000. I asked why MTN has not contacted me before handing over the claim and that I did not know about this account. He then indicated that MTN had tried to contact me on the addresses that he informed me and I informed Thulani that i did not know about these addresses. He then requested for my email address which I provided. When i saw the account number from the email and when I called the debt collectors I realized its a case of mistaken identity. I informed Thulani to go back to mtn and get the correct contact details since I am not the person they are looking for. Now he said i should go to the police and get an affidavit stating that i am not Mary Sekai. Why should I now start running around to go to police for an account that i do not know of?? Why should i start spending money to go to the police for an account that clearly belongs to another person. This lady has different names from mine, she is Mary Sekai, I am Mary. The lady has different ID numbers, date of births, different contact details, etc. I am not sure where Thulani got my number though because MTN has never called me regarding the account because MTN knows that the account is not mine. The problem is Thulani keeps calling and harassing me for an account that i do not know about, to make matters worse sometimes he calls me when i am with my colleagues at work and now everyone knows there is a debt collector who is stalking me. He keeps sending me emails and I have persistently informed him that he should get the correct details of the person who owes on the account from MTN. I find this very unprofessional. I am very annoyed by this behavior from a debt collector and wonder how many people out there are being harassed for accounts they do not know of.