Time of supply for spectrum services: GST arises on due or actual installment payments under reverse charge. GST on spectrum allocation by government is payable by the telecom operator under reverse charge. Where deferred instalment payments create a continuous supply, the time of supply is the earlier of payment (entered in recipient's books or debited) or the date immediately following sixty days from issue of an invoice; Frequency Assignment Letters are bid-acceptance documents and invoices must be issued on or before ascertainable instalment due dates. GST is payable on upfront sums when paid or due, and on instalments when due or paid, whichever is earlier.