Correct me if I'm wrong... But just because they keep your FDP clock running, you have no obligation to answer the phone if they call. If you finish your last leg on your last day, with 6 hours remaining on your FDP, and 15 minutes after block in you've heard nothing you're free.
You're not on reserve, and according to 117 you can't be put back on reserve until you've been given 10 hours rest. Some guys don't agree and they consider themselves on the hook for the remainder of the day, sit at the airport, missing all their flights home for the day. I don't think that's right.