I am using a unique analog telephony device that is connected to a VIC2-2FXS Voice Interface Card on a Cisco 3725 router. I noticed that it does not always hang up correctly, and this problem does not occur with other Voice Over IP adapters.
I decided to measure the voltage between Tip and Ring on the router's Voice Interface Card, and here is what I got:
On hook: 46.1V
Off hook: 6.26V
I noticed that it switches the polarity between when you're off hook and when you're talking, whereas other Voice Over IP adapters usually have about 6.26V for both.
So my question is, is there any way to disable this polarity switching? I suspect this may be part of the problem I'm seeing with incorrect hangups.