Unknown debit

When I was busy with a transection yesterday evening I realised that an amount of R4234.79 was debited on my account by some MTN SP which I know nothing about, that money was reserved to be used this weekend and now I don't know what to do, tried to call them last night and I was on hold until I ran out of airtime, I'm very disappointed with my bank because these incidents keep repeating themselves and I'm expecting to be reimbursed for all the extra costs that I'm going through because of their mess......