What to do when a device doesn't specify it's DC polarity?
Fixing telephone polarity
RJ11 Telephone · AllPinouts
What's the big deal about reversed polarity?
Phone Line Polarity Checker
You could even re-joint the cable inside the shared connection box if you were feeling confident.
This wire can be any color besides white or green, but it's usually black or red.
The primary line will use the red and green both solid lines.
You don't need to strip the wires.