Jonny,

Its not the resistance, its the impedence of the transformer.

Anyway to use it you may try as follows:

Pickup the handset of your normal telephone (make it off hook) and measure what is the DC voltage at the line after off hook.

Now connect your transformer accross the line and measure the voltage. If it is less than normal offhook then try adding a resistor of 2 watt in series of the transformer.

Start from the values from 200 ohms up to 800 ohms, fix the value where you get the approximately correct voltage drop.

I have tried this and works OK, though not an engineers approach, but just to go around impedence matching hassle.

Be carefull here, too low of resistance will give a signal loss and you may not be able to send/receive DTMF all the times. If it happens just add a 100 ohms more.

Ideally your transformer impedence should match the line impedence. Use an impedence meter.

But that strikes a question, I have used a variety of modems in my PC with my home telcom line. Every transformer in each modem has got different "resistance" but all work fine :-o)

A good commentary from a Telcom engineer is invited please.


Simon