The C-1000A/B is the controller and can be used with a number of Viking products as well as an ordinary analog phone (see
Section H of the C-1000 manual for details).
Two thoughts:
1) Program the CO port on a phone's button (or dial #9 and the trunk number). If you then dial #1, can you hear the person at the doorphone (this won't work with a standard 2500 set)? If it IS a standard 2500 set, what happens when you dial x23?
2) If that doesn't work, I wonder if you need to start from scratch and default the unit. I'd sure like to know what's hooked up to what and the positions of all three dip switches as well as the Talk Battery switch.
If you get tight on time and can't work with the delay of the back and forth on the board, call 1-715-386-8666 between 8:00am and 4:00pm Central for free technical support.