Quote Originally Posted by CurtZHP View Post
That's the likely culprit. It works, but it's not necessarily consistent. Usually, all it takes to fool the hybrid is for the company's gear to insert the ring-tip voltage on the "POTS" line, because that's all that 99% of end-user equipment needs.
Sidebar: I'd always thought of the ring conductor on a TRS connector to be named that because of its shape (much like "tip" and "sleeve"). I have an engineer friend who told me that the name actually comes from the voltage applied to that conductor used to make the phone ring. The "ring voltage."

Apocryphal?