Telecom nerds: I was recently reminded of a strange wiring fault at my last house that I never solved. I live in another city now so it's way too late to troubleshoot but I'm curious if anyone's ever seen something like this before or have any ideas as to how it happened.
Setup:
* Comcast DOCSIS to POTS VoIP bridge in garage
* Short RJ11 cable from VoIP box to demarc box (bypassing the original Centurylink POTS wiring to the pole) bridging to existing building cable plant
* In-building wiring from demarc point to wall jack
* Analog phone in house plugged into wall jack
Symptoms:
1) 48V DC on-hook bias looks fine on voltmeter
2) Phone gets power and dial tone, can make outbound calls without issue
3) Inbound calls will connect if you magically pick up the handset at the right time, but the phone does not ring
4) No 90VAC ring voltage present when probing the line and dialing it
5) After running a new wire directly from phone to the VoIP box (bypassing the in-wall wiring), everything works fine. So the fault is definitely the in-wall wiring and not the phone or VoIP box. But how can you short 90V but pass 48V - and even <1V analog audio - OK?
My best guess is that it's a damaged conductor that is able to block 48VDC OK, but 90VAC is enough to arc and short. That's a narrow window but it's physically possible. Anyone have a better theory?
Did you check for ringing voltage with no phone or any other load connected? I assume you didn't check for ringing voltage on the VoIP box side when connected to the old wiring?
@robryk No ringing voltage on the socket with phone unplugged. Did not check on the VoIP box side, that would have been interesting in retrospect.