Excellent advise all...thank you.

Gunner, that makes perfect sense, and I did find an open line marked '3' on
the brain box. It looks like they just have a regular phone cable running
from the main analog incoming line from outside, to the brain box for phone
line #1, and then a line coming from the doorbell speaker box to line #2. I
guess my fear now is that the system will need special programming (like you
said) to recognize the new line. OK, I've built up the courage, off I go to
find out.

Ryan