freeswitch/conf/testing/dialplan/default/0008_eavesdrop.xml

<extension name="eavesdrop1">
  <condition field="destination_number" expression="^6020$">
    <action application="answer"/>
    <action application="eavesdrop" data="all"/>  
  </condition>
</extension>

<extension name="eavesdrop2">
  <condition field="destination_number" expression="^6021$">
    <action application="set" data="absolute_codec_string=PCMU"/> <!-- MONO -->
    <action application="answer"/>
    <action application="eavesdrop" data="all"/>  
  </condition>
</extension>
<extension name="eavesdrop3">
  <condition field="destination_number" expression="^6022$">
    <action application="set" data="absolute_codec_string=OPUS"/> <!-- STEREO -->
    <action application="answer"/>
    <action application="eavesdrop" data="all"/>  
  </condition>
</extension>
<extension name="eavesdrop4">
  <condition field="destination_number" expression="^6023$">
    <action application="set" data="absolute_codec_string=OPUS,VP8"/> <!-- STEREO, VIDEO -->
    <action application="answer"/>
    <action application="eavesdrop" data="all"/>  
  </condition>
</extension>
<extension name="eavesdrop5">
  <condition field="destination_number" expression="^6024$">
    <action application="set" data="absolute_codec_string=PCMU,VP8"/> <!-- MONO, VIDEO -->
    <action application="answer"/>
    <action application="eavesdrop" data="all"/>  
  </condition>
</extension>