Airtel — My mother's Airtel no is registered in someone else name

My name is Srikrishnan. We purchased this no. almost 15yrs back and we have been the sole user of this no. I even registered this no with mother's aadhar. Recently I purchased a 4g mobile for my mother and we tried to get 4g sim. But the customer care said that this no is registered in someone else's name. We have been the sole user of this no and I don't know how this happened. We want this no and we request Airtel to assist in necessary actions to get 4g sim for this no.