Thanks for responding, Franc and Lee. I can see how a voltage test might
give me some clues about the pinouts and which standard(s) may be in use.
Lee's comment about an inline module is interesting, and partly on account
of voltage. One of the old AT&T pieces that I tried to use also had such a
module. I believe it has something to do with voltage support for the two
sides of the connection. If that turns out to be essential then my
do-it-yourself project may become overly complex. But I'm probably not
going to buy a $30 add-on for this old unit.

--John Hupp