Okay, here's a dialplan snippet that I used to successfully do the autocallback. In my case I used ext 1001 as the customer and portaudio as the "agent" if you get my meaning. Extension 1001 dials 9902, hangs up, and immediately the api_hangup_hook's originate command is executed. In this case it calls portaudio/auto_answer for the A-leg and user/1001 as the B-leg. I don't claim that it's the prettiest thing in the world but it definitely works. You'll need to adjust according to your specific situation.

  <extension name="callback-test-answer-ib-call">
    <!-- From mailing list - a question about how to do this:
        Caller calls in, ring (no answer), capture Caller ID, wait for caller to hangup
        Generate outbound call to captured caller ID number
        Only use dialplan, no scripting
    -->
    <condition field="destination_number" expression="^9902$">
      <action application="pre_answer"/>
      <action application="set" data="callbacknum=${caller_id_number}"/>
      <action application="log" data="INFO Callback number is '${callbacknum}'"/>
      <action application="set" data="api_hangup_hook=originate portaudio/auto_answer CBTEST${callbacknum}"/>
      <action application="sleep" data="10000"/> <!-- wait 10 sec for caller to hangup, otherwise we hangup -->
      <action application="hangup"/>
    </condition>
  </extension>

  <extension name="callback-test-generate-ob-call">
    <condition field="destination_number" expression="^CBTEST(\d+)$">
      <action application="bridge" data="user/$1"/>
    </condition>
  </extension>


Let us know how it goes. BTW, what is the reason for this type of scenario? Just curious.
-MC