Thanks for the response.  How did you know I have a Princess phone?  ;)

But note he actually HAS 2 land lines and is thus USING two pairs. I
think the Cidco is using an unused pair to backfeed the cellular to the
extension phones in the house (their install manual is horrible)--they
appear to be using a small inline converter/splitter at each extension
to access both the original line and the cellular line.  An unusual
method--the other brands feed both signals on one line but need to be
installed upstream from extensions--but it would allow simultaneous use
of land line and cellular.  According to their doc if you already use
both lines you can't use extensions--you can only use whatever is
plugged directly into the aux port at the base--they recommend a multi-
handset cordless system in that situation.

Actually there are other options.  Like you said newer houses may be
wired with 4 pairs with two pair not connected to the jack.  If he can
access the outside box he could rewire it moving the work phone to
another pair and only at the work phone tap into that pair.  Otherwise
every jack would have to be rewired. If it's only 2 pair wire AND he can
get to the box the work phone could be taken off the 2nd house pair and
a direct wire run to the work phone. A 3rd option might be to drop his
personal line and only use cellular--that would free up a pair.

Pretty complicated for a non-technical person--the OP might need to hire
a professional.