Airtel telecom distributor gst return

Hi Everyone please help me out from this dilemma One of my client is telecom distributor of airtel. He is issuing commission invoice to airtel company and paying GST on commission. Then whether he is required to issue outward invoice for each recharge value transferred by him to retailer. Airtel is issuing tax invoice for each value order placed by distributor and it is appearing in gst-2A of distributor. Please provide you expert view on given issue. As per current position, distributor is only issuing commission invoice to company charging gst on the commission give by airtel. and please tell me how to file his GST return on which basis on turnover which he sale or on commission basis Please tell me the process of GST returns.