Didn't your company try to get pilots to check in for their flights hours before from a local phone number some years ago?
If you "deviate" and don't take the scheduled deadhead to an outstation to start your flight, you must get there ~12 hours before show time and call in to skds.
If you take the scheduled deadhead, you are leaving out of your base, and thus must check in at the ops center before heading to the pax terminal.
I would hasten to add that I don't always give myself 12 hours prior to showtime when I jumpseat in. I find that 4 - 6 hours in a comfy recliner at ops is more than adequate to start a trip that leaves at 3 am - even a long one.