Quote:
Originally Posted by julie_m
conceivable that a high-REN phone in parallel with it could pull the voltage down just too low for the Zener diode to start conducting. .
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The voltage dropped across an unmodified 312 bell (or any 59A bell movement phone) is greater than the voltage dropped across a modified (bell plus 3k3 resistor) telephone: for a 75V 25Hz supply, say, 89V and 78V respectively. But this is without the extra loading of the high-REN answerphone, which I wouldn't have thought to be that severe a load anyway.
I was going to suggest the ringing voltage might be too high, but the spec says 100V down to 40V, so any BABT equipment should be OK.