You are on duty sitting short call reserve. If you are sick, you need to call out at the time you are no longer fit to fly, not after being assigned a trip. Granted there’s always exceptions but it’s pretty obvious what’s going on. Either the pilot is not in position to fly (commuter) or they don’t want to burn up sick days unless they have to. Or in the current environment they’ve got a CJO and simply want to spend their days skiing instead of this silly, annoying job.