Disappointed client

I have been a customer for MTN for almost 20 years and I took a contact with them for 2 years and paid it in full but recently , I noticed that they were deducting amount from account and reversed it as I didn’t know what is was for. Now my number is blocked and I cannot make calls or receive calls . Call them their call center and was advise that I need to pay money that I have no idea what is it and they will unblock my number . I don’t need this now as I need to use my number. I’m really disappointed in them cause they couldn’t help me and being a loyal customer of theirs for so long.