to the BB.
It makes no difference if it is a Nortel or Avaya...slt (pots) is pots. Once it hits the traveler cable at the elevator equipment (your Demarc to the elevator) ....it's his. If you test it at the Demarc with a butt set and it works properly, not loading the E911 trunks, it is his.
You have done the tests. Even with the other dialtone it still loads the E911 trunks does indicate a problem within his unit.
To isolate it further, test it from the point the wiring goes into his dialer inside the cab.
Now then, before I get caught with the statement about pots is pots, let me digress. Pots lines, depending on the switch can either be 24 or 48 volts nominal. In some instances CPE equipment can act up if it doesn't like the line voltage.
Then there can be polarity requirements on some CPE units. Rare but possible. Although it shouldn't make a difference. But who knows for sure?
:shrug:
Good luck and enjoy the BIG State !!!