Calls received for account

This week for three days (8th, 11th, 12th) i have received a call from +27 ***. Unfortunately the callers always begin with verify your ID which i don't provide in this day and age to any caller i do not know, due to ID *****. First call on 8th advises i owe R9 725, i advised i am up to date on all payments - she asked to provide receipts, i advise i pay by Debit order on the system. Caller would send me a statement - received nothing. Second caller Sharlan 11th asked for my date of birth, i asked for the query, she advised if i don't provide DOB she'll terminate call, she did. Third call today Sibu says they are from collections and will send me a statement - that was 11h28 - still nothing. I have been advised by MTN locally i owe nothing - have checked on MTN site, and i owe nothing. I have been advised call centre is striking and no one there to make these calls so i am extremely suspicious and would like an email from MTN from an official site advising what is going on please? sent email to MTN customer care, with a read receipt which was read at 16h48 yesterday with no response either. 20 years with MTN and first incident which is not a pleasant one at all.