AT&T Merlin analog multi-line telephone protocol docs?

[5415551212]

Does anyone know anything about the AT&T Merlin Signaling protocol that the 206 / 410 etc units used?
"analog multi-line telephone protocol (40 kbps)"
According to wikipedia

"The Merlin system introduced a new wiring design of eight wires (four-pair) that allowed the system to deliver two voice paths to each desk phone so a user on an active telephone call could still receive a paging announcement on their phone's desk speaker simultaneously. AT&T described the signaling channel on the data pair as "analog multi-line telephone protocol (40 kbps)" but no further details were made available about the protocol"

I just got a Merlin 206 that seems to still work and it would be fun to make a board to tie it into a key 1A2 system.