Using multiple ML17939 phones with the same lines.

Im setting up a small office for a friend. They only require 3 phones and they have 2 phone lines. Im planning on using ML17939 phones but I cant seem to find a good description of how these phones will work together. I read comments about them communicating over line 1 digitally with each other but I have yet to find some information on what that means exactly. Does anyone have any information about this communication function? Thanks in advance.

Theres no such thing as too much detail, im curious HOW they work as well, assuming they do.