I feel like there needs to be some regulatory recognition that "on call" is neither on duty nor on rest, especially for EMS. When I'm on call, I'm generally sitting around doing jack, but I can't really get too far into doing anything that's going to take along time to clean up and be flight ready for (sleep included) lest a call come in. What would really be nice is a reg that would say somthing to the effect of:
You can only be on call up to 12 hours in any 24 hour period, and those 12 hours must be contiguous (i.e. no "off call for the hour after you've landed so you can postflight" BS) and preceeded by at least a 10 hour rest period free from duty assignments or any obligation to accept a duty assignment,
You are eligible to accept any duty assigned within that 12 hour period provided you complete an operation covered by part 135 no later than 14 hours after the beginning of your assigned on call period, and
Your rest period does not begin until the aircraft has either returned to base and all flight-related duty is completed, or the flight crew determines that due to fatigue the airplane cannot safely be returned to base at which point no duty may be assigned or accepted until the completion of a 10 hour rest period. <-- This one would be hard, because I could see some of the more shady employers applying fatigue as a punitive circumstance.
If a rest away from base is required, the pilot may be considered "on call" upon returning to base until no later than twelve hours after the beginning of that flight duty period, and eligible to accept assignments provided the on-demand portion of the flight is completed no later than 14 hours after the beginning of the duty period
8 hour flight time rule would still be applicable.
I feel like you could still run a two pilot operation smoothly using these criteria because the 2nd pilot's on call would not begin until the flight returns to base, at which point they would be on call up to twelve hours but the other pilot would only need 10 hours rest. I can't imagine a scenario under normal EMS operations where neither pilots could accept the duty assignment, unless at like 9.5 hours into the first pilot's rest (and therefore 9.5 hours into the second pilot's on call) a stupid looooong call comes in, and even then in that scenario a delay of 0.5 would typically be tolerable.