You don't have to necessarily enter a receipt number at all. I was only issued my I485 receipt number in March 06 and I have been calling since I applied in August 04. Just press the option for not having a receipt number and you will eventually pass through. Alternatively, from the main menu, select 2 and then 1 which will take you to the change of address option which connects you straight to a customer service agent, thats what I usually do.