You don't need to acknowledge a short call until one hour to the start of the short call.

6. A long call pilot who is assigned a rotation or converted to short call:
a. prior to release from a rotation, must acknowledge such assignment or conversion prior to release.
b. via telephone contact or electronic placement on his schedule on his last non-fly day (other than a vacation day) before an on-call day, must acknowledge such assignment or conversion no later than:
1) three hours before the scheduled report of the rotation, or
2) one hour before the scheduled start of the short call period.

The More you Know.