We have FOP2 version 2.27 running on a Debian Asterisk PBX (v 1.8.13.1) for our office. Any time we try to use the Listen feature in FOP2, we can only hear one side of the conversation. It doesn't matter if the call is local or if it's going through our SIP trunk, we can only hear the audio being transmitted from the extension we selected before clicking "Listen".

Nevermind, I found the issue.

For future reference (and for others with the same problem), edit /usr/local/fop2/fop2.cfg and change "spy_options" to "bq". It looks like this might be the default, but we had it set to "o" for some reason.