Impedance matching really is an issue when your source is 600 or 900 ohms and your amplifier is 10K Ohms. The matching is done to reduce hum and get the signal as close to intelligible as possible.
I disagree. Gone are the days when we matched the source impedance to the load. Today it's a low impedance into a high impedance and that's both for an unbalanced and balanced line. It matters little what the source impedance is as long as it's lower than the load, all that matters is that it is at a suitable level that the particular amp input wants to see.
BUT for long balanced lines it is best to use transformers at each end that will provide good balance and an impedance match to each other. Then, yes, you do that to minimize or eliminate hum pickup and other common mode noise.
The 600 ohm impedance is for audio out of the FXI that gets connected to the paging amp input which is generally something like 10k. Nothing to do with the phone line.
-Hal
