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try this
Classic 500s had a potted transformer type hybrid network. 500s closer to the end of production (70s-80s) had a small solid state circuit board. I wonder if the wall set you have is possibly the newer style and therefore draws less current. Maybe swapping hybrids from a newer 500 might help.
Also, of course when dealing with old phones and VOIP type systems, it helps to connect only one old phone at a time. Even when hung up, a 500 or similar phone exhibits a weird capacitive/inductive load due to the mechanical ringer.
Al
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