Phone Voltage
On 03 Apr 2012
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The phone ring is usually 20 Hz, but 60 Hz will work fine as you note. When a phone is taken off-hook, the switching equipment at the telco premises sees this and removes the ringing voltage an applies 48 VDC for the voice part. The phone impedance off-hook should be about 500 ohms. On hook is about 1200 ohms if I am remembering right. If the phone rings, you no longer need to keep the 90 VAC applies. Remove it and connect the 48 VDC. You need to watch your polarity. One way will work and the other way won't. If the impedance changes correctly, and the audio works, the phone should be OK.
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