Turn your question around.  What is the benefit of tying up a CO just for the Doorking if you don't need to?

Doorking has the intercom mode so you don't have to tie up or dedicate a CO (phone line) to the 1812.  The phone system still sees it as an incoming line.  Read the manual to learn the codes for operating the 1812 from a system phone.  Program system phone DSS buttons for hold open, timed open, close, etc.  Much easier to do this with the 1812 in intercom mode.

If you didn't have the phone system then you'd need to loop a CO through the 1812 as you mention in your first post.  But since you have the Pany 824, intercom mode is the way to go.