Tom,

You wrote:
"How can I tell which is the tip and which is the ring? " (or A & B legs in
NZ/UK terminology)

The answer is, as you have already found, that one is at -48V (or a little
higher) when the phone is on hook, i.e. drawing no current.

This is however somewhat irrelevant. The DC component is only used for
remote powering of the phone, and basic signalling. The ADSL modem is an AC
(>30 kHz) device and doesn't care about polarity. So connect it either way
and it will work. The important thing is to get the filter correct, i.e. low
pass side to the phone, high pass to the ADSL modem.

Steve E

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