It's not regular reserve like 4 to 4 or 9 to 9. Your callout period is only your originally scheduled duty period. And they can only assign flying during your original duty period. So if you were scheduled to work 10am to 7pm, they can call you at 10am for a noon show and it must release by 7pm. And you'll get at least the credit that your original pairing paid.
I've had stuff cancel out like this in the past and I never wound up working. Granted I typically fly locals or split duties and don't know how it would play out on a 3/4 day.
But like I said, read up on it so they don't use you outside of policy. My guess is they will "reflow" some into trips out west. Watch the report/release times and the credit.