RJ 31X Alarm Jack wiring

RJ-31x is tricky to understand if you're not familiar. Looks like an RJ-45 but behaves much differently.
These are used in the United States to provide dialtone to alarm panels.

The RJ-31x is special in that it has a mechanism inside the allow for sharing of the line with the customer's PBX equipment. However, the alarm panel still has priority control over the line. Even if the line is in use, if the alarm panel needs it, it can seize control of the line, disconnect the customer and then use that line to immediately notify the monitoring center.