I know phones are capable of operating off hook without separate power sour ces, but I was wondering if it might be possible these days to power a phon e/answering machine entirely off the phone line. So many require AC power and fail to operate, might it be possible to vampire enough power from the phone line so a cordless phone/answering machine might work from the phone line alone?
I had a little trouble finding any reference specifically indicating what p ower level draw is allowed on a phone line. I did find a Remote Subscriber Terminal POTS equipment spec from Tellabs that specified it would detect o n hook at 9 kohms. It also specifies a 25 mA loop current at 600 ohms whic h I take to mean in combination with the internal current limiting resistan ce in line with the 48 volt PSU. Doing the math I figure the in line resis tance is 1,320 ohms which gives a 10 kohm load up to 4 mA at around 42 volt s and 180 mW.
That sure seems like a generous budget to keep a phone charged even if not in real time operation.
Is there a regulation for phones that precludes this?