I found a remark about 'connecting a device to M3904' in the M3904 Quick Reference Guide, it's worth a try;
Activate Headset port external alerter:
When the Headset port external alerter option (available on the M3904) is activated, it supports a visual or audible alerter and/or recorder device. This device plugs into the Headset port, which is located on the bottom of the telephone.
After you select Headset port ext alerter from the Options List menu;
From the External alerter screen, press the On Key to turn on Headset port external alerter, or press the Off Key to
turn off Headset port external alerter. Press the Done Key to save changes. Press the Quit Key or Options Key to exit.