If they are so busy they can't return a call with-in an hour or two or answering email within 24 hours perhaps a 2nd customer service person might be worth the salary. If the factory can't keep up with all the orders then the lead times for product need to be extended. People understand if CS tells them that it will take longer because there is a queue of orders to be filled before theirs. If it's a problem they can decide not to wait and order else where. Keeping the client informed will preserve them a a potential client in the future. The other way you just turn them off and it's doubtful you'll ever get a order.