With all of those willing to volunteer in the park in the shops and offices as well the trails, surely they could expand the number of hours that the reservation phone lines are manned. Were they to find enough people to volunteer 20 half days per month, the lines could be active for the entire business day five days per week. That would effectively double the amount of time that reservations are taken each month. The calls are inbound, so essentially the service could be doubled at no extra cost.
I normally volunteer my time to do trail maintenance in some of the surrounding areas, but I'd be willing to take on a half day per month to save thousands of hikers the expense and hassle of reserving all of the back country sites.
The primary problem is the shelters and the sites requiring reservations, the vast majority of the sites don't even need to be included in this were it not for the added revenue that they might generate.