Michael,

what you want to achieve is pretty simple to do with FreeSWITCH, and is quite close from what I do.

Or you do like me: you split calls coming into public to a specific contex, based on source IP with such extensions in public.xml:

  <extension name="from_provider">

    <condition field="network_addr" expression="X.X.X.X"/>

    <condition field="destination_number" expression="^(.*)$">

      <action application="transfer" data="$1 XML provider"/>

    </condition>

  </extension>

  <extension name="from_sipxecs">

    <condition field="network_addr" expression="Y.Y.Y.Y"/>

    <condition field="destination_number" expression="^(.*)$">

      <action application="transfer" data="$1 XML sipxecs"/>

    </condition>

  </extension>

And you just have to add the right extensions in those dialplans to bridge calls to the other side.

That's the easy part.

<extension name="provider">
  <condition field="destination_number" expression="^(3015551212)$">
    <action application="bridge" data="sofia/gateway/sipxecs/$1"/>
  </condition>
</extension>