frege:~# asterisk -rx 'dialplan show tests'
[ Context 'tests' created by 'pbx_config' ]
  '220' =>          3. SayUnixTime()                              [pbx_config]
  '221' =>          3. Festival(hello world)                      [pbx_config]
  '222' =>          3. MusicOnHold()                              [pbx_config]
  '224' =>          1. TestServer()                               [pbx_config]
  '226' =>          1. Playback(20sec)                            [pbx_config]
  'fax' =>          1. NoOp()                                     [pbx_config]
                    2. Answer()                                   [pbx_config]
                    3. Monitor()                                  [pbx_config]
                    4. RxFAX(/tmp/test-result.tiff|debug)         [pbx_config]
                    5. Hangup()                                   [pbx_config]
  'faxsend' =>      1. NoOp()                                     [pbx_config]
                    2. Answer()                                   [pbx_config]
                    3. TxFAX(/tmp/test.tiff|caller|debug)         [pbx_config]
                    4. Hangup()                                   [pbx_config]
  '_225.' =>        1. TestClient(${EXTEN:3})                     [pbx_config]
  '_22[012]' =>     1. Answer()                                   [pbx_config]
                    2. Wait(1)                                    [pbx_config]
                    4. Hangup()                                   [pbx_config]
frege:~# asterisk -rx 'originate Local/faxsend@tests extension fax@tests'

I get two channels running, and nothing seems to get recorded in the monitored RX channel:
frege:~# ls -l /var/spool/asterisk/monitor/
total 8
-rw-r----- 1 asterisk asterisk 44 2007-11-26 14:55 audio-in-0.wav
-rw-r----- 1 asterisk asterisk 44 2007-11-26 14:55 audio-out-0.wav