Peter,

I once worked on stuff that operated on leased 4 wire lines in the frequency
range of a few KHz to about 200 Khz. We were told by Telco systems engineers
to design for 135 Ohms. There were restrictions on what cable was usable. 19
ga was OK, 26 was definately not. Loading coils had to be removed. I think
it was called a "G" something conditioned line.

There is an FCC approved model for a telephone line input impedance at voice
frequencies. I can't remember the details, but it was something like two
resistors in series with a sum of around 1000 Ohms; the larger resistor
being shunted with a fair sized capacitor. You need this model to design,
for instance, a hybrid for a modem.

When the line is very short relative to a wavelength you could treat it as
something other than a transmission line. For instance, I once used a 1 inch
length of RG 58 as a neutralizing capacitor for an HF traansmitter.

Tam/WB2TT