Da Voismart Knowledge Base.
Monitor a channel

Used to start monitoring a channel. The channel's input and output
voice packets are logged to files until the channel hangs up or
monitoring is stopped by the StopMonitor application.
  file_format		optional, if not set, defaults to "wav"
  fname_base		if set, changes the filename used to the one specified.
    m   - when the recording ends mix the two leg files into one and
          delete the two leg files.  If the variable MONITOR_EXEC is set, the
          application referenced in it will be executed instead of
          sox and the raw leg files will NOT be deleted automatically.
          sox or MONITOR_EXEC is handed 3 arguments, the two leg files
          and a target mixed file name which is the same as the leg file names
          only without the in/out designator.
          If MONITOR_EXEC_ARGS is set, the contents will be passed on as
          additional arguements to MONITOR_EXEC
          Both MONITOR_EXEC and the Mix flag can be set from the
          administrator interface

    b   - Don't begin recording unless a call is bridged to another channel

Returns -1 if monitor files can't be opened or if the channel is already
monitored, otherwise 0.