Check Essential
From ALPA's Guide to 117:
Q-32. A flightcrew member is on days off. Can the company call him for a flight assignment?
A-32. Yes. If the flightcrew member answers the call, the
company can assign him to flight duty. However, the
Company must give the flightcrew the required 10-hour
rest period and then place the crewmember on short call
reserve.
Q-33. Can a flightcrew member who is on days off voluntarily elect to
pick up a trip from open time?
A-33. Yes. This is not prohibited by the regulation so long as the
flightcrew member is rested, fit for duty, within applicable
limits for duty and flight time, and prepared to perform the
assigned duties.
I would think 32 applies to inverse assignment (ie: they're hosed.)
I would think 33 applies to GS/WS.
My opinion only.