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Light show and Phone power questions

Tha fios agaibh

Aug 11, 2014
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Last weekend an ice storm caused a tree to fall directly across a 15kv utility line in our yard. After an impressive light show, about 700 of my neighbors and I were left without utility power for a day.

My backup generator kicked in a minute afterwards to give us household power, but there are a few circuits in our house that are not tied in to the generator.

I checked the phone and it was dead.
(Given, this is an older style wall phone that have keys that light up when the receiver is lifted).
I then ran an extension cord to the cable modem in my office thinking that would power it up since its a voip data system.
Still dead.
Utility power came back the next day and all returned to normal but I was left with a few questions.

What supplies power to the phones if not the cable modem itself? Is it like a poe system where there is a external power supply injector that powers the phone wires?
The cable modem even has a backup battery that's supposed to keep the phone working when power is lost.
I imagine that would only work with an old phone that doesn't require (48v?) power?

I understand that the phone would not work if the isp/cable provider was down, but I expected my old phone to at least light up when I picked it up.
 

kellys_eye

Jun 25, 2010
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ISTR the POST (plain old telephone system) uses a 50V (or thereabouts) supply on the line. It is AC (for ringing) and DC for conversing. If the local switchboard (exchange) went down then it should normally have it's own battery-backed supply to maintain that voltage.
 

Tha fios agaibh

Aug 11, 2014
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....its a voip data system.

What supplies power to the phones if not the cable modem itself?
Thank you Kellys_eye, but it doesn't answer my question.
It's not a traditional phone (Post)system except the original field wiring in the walls.

There is a phone jack on the back of the cable modem that mimics / interfaces a traditional phone via voice over internet protocol. (voip)
Assuming it would need the ~50v to power and ring a traditional phone, Where does its come from? Internally in the modem, or an external power supply hidden elsewhere in my house?
 

Joseph_b

Apr 19, 2018
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the cable modem is providing the voltage needed to ring a traditional telephone.
if the ISP was down it might not allow the unit to operate dialup modem.

essentially this works just like a PC telefax modem.
there is a DSP to generate and translate DTMF tones and code/decode voice,
and there is a power supply and a transformer to step up to line voltage.
 

Tha fios agaibh

Aug 11, 2014
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Thanks Joseph_b, I was just expecting power sent to my phone (illuminated dial) when the receiver was picked up provided the modem had power.
Perhaps it needs to communicate with the isp before the rj11 jack outputs power to the phones?
I suspose that could be tested by simply unscrewing the incoming coax cable and see if phone illumination turns off.
 
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