Over R4000 in unexplainable arrears

My retired mother, after over 20 years, using her number suddenly got disconnected from the network although she's never missed a payment on her contract. When she went to do a sim swap at the nearest dealership, almost 70km away from home, was told she owes over R4000 but none of the consultants could explain how it was accumulated or what the amount is for. She's still stuck without her number for over a month now and MTN is failing her royally for the second time in as many years.