You’re going in the right direction. And like you said, the panel only normally checks for phone line voltage, not necessarily a physical dial tone. Once a day, the panel will also attempt to call the central station and send a test report to confirm communications. If any of those things go wrong - loss of voltage on the phone line or incomplete communication attempt, you will get your trouble codes. The DL and DF faults are going to happen if the panel attempts to communicate with the central station and the transmission did not go through - this could be due to an interruption during the call or noisy phone line. The L1 and L2 faults are going to happen with a loss of voltage to the phone line.
First thing I would check - are the phone lines wired correctly at the d-mark. The fire alarm panel should be wired as the FIRST device on the phone line, and wired in such a way so that when the panel needs to communicate, it can disconnect all other telephones on that line. The fire alarm panel phones lines should go right to the d-mark. Let’s take your average Cat5 cable, blue/white pair would be line #1 and should land directly on the d-mark ring & tip terminals (feed). These should be the ONLY wires on the terminals! At the panel, the blue/white pair should be on the red & green terminals on the RJ31X. Then the orange/white pair at the d-mark should then connect to the wires feed all the other phones on that line (return). At the panel, the orange/white pair should be on the grey & brown terminals on the RJ31X. Repeat for phone line #2 using green/white as the feed and brown/white as the return. Attached below is a diagram on how this needs to be wired.
Once you’ve confirmed both telephone lines are wired correctly we can proceed from there. Keep in mind some jurisdictions require at least one phone line be dedicated and you may not be “returning” anything to other phones - then only the red & green terminals on the RJ31X would need to be connected.
