In article ,
The Natural Philosopher writes:

Ther are only two wires which connect you to the exchange, and the way
round they are is not important.

That's supposed to be true for all phone appliances, and it
is for most (back in the days of GPO/BT approvals, it used to
be tested). However there are some that don't work properly
if the line is reversed. Some caller display units have been
the more recent culprits, including modems with that function
built in (Hayes fixed it in later models of theirs).

Identify those, extend them and put your own master box in.

--
Andrew Gabriel