Mike, check out
THIS thread that deals with how to use a relay with the M1TWA for announcements both from Elk and a home automation system. I use this system to input announcements and .wav files from CQC into my three M1TWA zones.
Post number 8 in that thread details the pin out of J1/J2 and J3/J4 and notes from ELK customer support that "
When any or all of the three outputs (Out7 = J3/J4-pin 12, Out8 = J3/J4-pin 11, Out9 = J3/J4-pin 10) are ON and output 10 (J3/J4-pin 9) is OFF, the signal from the mic(s) will be present on J1/J2-pin 7."
I'm in way over my head with this suggestion, but since the mic signal(s) are evidently present on J1/J2-pin 7 when one of the outputs is turned on (and output 10 is off), I wondered if it would be possible for you to tap into J1/J2 pin 7 and send the mic signal(s) wherever you want to use the listen in function. I don't know if you would need to amplify the signal since I would assume that the signal goes from the M1TWA to the M1 where any amplification for the telephone connection takes place.
It might be worth a try depending on how you want to use the listen in function of the mics other than by telephone.
Good luck!