Airtel — problem faced due to recycled sim card issued

I have recently purchased new sim card with number +[protected]. The sim card issued to me was recycled one means it is already used by somebody else. Due to this i am getting unwanted sms and calls. I requested to issue fresh sim card and number. The request refused by airtel store stating new number are only assigned to to postpaid customers (This was not informed while purchasing sim card). Escalation was done via email with nodal office to resolve issue was also rejected. This is discrimination with customer. Requesting your support for getting fresh number