5) Attempt to balance the hybrid at the 2-line to 4 line interface.
Why: 99% of the time, this is where the echo originates and this is where is should be fixed. Unfortunately, this is not for the faint of heart, but if your line card has a hybrid balance adjustment (many don't), use it. Also, with multiple simultaneous calls, this may be the only real solution. Part of the problem arises from the use of lower impedance telephone wiring nowdays. The typical characteristic impedance of Cat5 twisted pair is about 100 ohms and many line cards are optimized for a 600 ohm line. This is made worse if the DC resistance of the wiring to the CO switch is relatively low. I haven't tried this myself, but you might try something as simple as a 500 ohm variable resistor in series with the ring line and adjust for minimum echo. If it gets worse, you haven't lost anything, just take the resistor out of the line. If it works, measure the value of the resistor when set for minimum echo and replace it with a fixed value resistor.
Tweaking the hybrid is really a waste of time. Most don't permit tweaking for this reason. Any change to the circuit, like changing to another phone (perhaps even of the same model) generally defeats the effect of any tweaking on the short lines of most PBXs. A well designed hybrid is fairly relaxed about termination, though the return loss can vary a lot across the audio band. Most approvals specs only call for about 12dB of return loss, and you will seldom see more than 20dB - even with hand tweaking. Whatever you do with the hybrid, only proper echo cancellation will clean things up well enough for good VoIP (or cellphone calls, which suffer similar high latency).
Actually twisted pair are generally below 150ohms. No line can have an impedance higher than 377ohms. 600ohm termiation is a fudge. On long lines the fudge doesn't work well, and loading coils are needed to refudge things. ADSL can't work with the loading coils in place, so they are being stripped out in many places. Such is life - messy!
Regards, Steve
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