If they put him on short call at 2030 he is now at rest until 0630 and has no obligation to answer the phone or acknowledge a trip until 0630. Scheduling is not normally willing to take a chance on when he might be able to report when they call him at that time. He certainly had no obligation to arrive at the airport prior to 0830 and it could be longer depending on traffic ect.. Trips often also bounce in and out of open time. They place a trip on a pilot schedule and he does not respond or calls in sick or the move him to another trip and that trip drops back into the pot. In general in the situation you mentioned they would GS the trip before taking the gamble on when the short call pilot can get to the airport.