Flipkart — Can't talk to customer care regarding an order of mine because the registered number is not active anymore.

The issue is regarding my order made for POCO M3 back cover on 12th October. I created my Flipkart account few years back, between then and now I lost my mobile and and lost the registered mobile number along with that. But every thing was working fine because I used to order things by giving my new number. Now there is an issue regarding one of the orders. The thing is I opted to return one of the item but the delivery guy is not coming to pick up the order. So to resolve the issue I wanted to talk to their customer helpline number but I can't call them because their process is calling us back and I think they are calling on my registered number which had been lost for quiet a few years.