The RT9080 Sub rings OK for loop ringing but does not ring Tip to ground or Ring to ground.

Because only the two center pins of the modular telephone are connected to Tip and Ring there is no ground connection to the phone for tip to ground or ring to ground ringing.

Remove the large interface board and, from the underside, Solder an AWG22 wire from pin #1 of the modular phone connector (the pin farthest from the LED's) to ground.