Is there a way to do this without the listen-in interface hooked up?
I am assuming you know that the listen-in interface will activate the audio monitor? You can then listen and speak through the house's speakers.
From the M1 Manual Appendix D:
2. Two way may also be initiated by calling in using the Telephone Remote Control, and pressing 7 - Audio Monitor
TONE COMMANDS
* 55 * BEGIN LISTEN-IN Used by Operator to begin a listen-in session.
1 TALK Talk through all speakers connected to Output 1.
2 LISTEN All 3 microphone zones are enabled initially. Use key 4 to change mics.
3 SENSITIVITY INCREASE Raise the microphone gain.
4 MICROPHONE SELECT 4+0 Selects ALL microphones, 4+1 Mic1 zone, 4+2 Mic2 zone, 4+3 Mic3 zone
5 [future use]
6 SENSITIVITY DECREASE Lowers the microphone gain.
7 EXTEND TIME for 3 minutes
8 - 8 TERMINATE SESSION and enable 1 ring callback answer for selected time.
9 - 9 TERMINATE SESSION without enabling callback answer mode.
0 [future use]
* CANCEL last key-press (undo )
# [future use]