Have the external profile be used only for provider ABC, or define a new profile. Then in the profile have the calls go to a specific context. You could have something like this in the sip profile definition:

<param name="context" value="abc_calls"/>

Then create a dialplan context called "abc_calls" that handles all inbound calls. Create a file in conf/dialplan/ called abc_calls.xml:

<include>
  <context name="abc_calls">
    <extension name="abc_calls">
      <condition field="destination_number" expression="^(.*)$">
        <action application="lua" data="myscript.lua"/>
      </condition>
    </extension>
  </context>
</include>

Essentially you're just creating a SIP profile and a dialplan context that are servicing your VoIP provider. You can add other profiles/contexts for other providers if need be.

Let us know how it goes...
-MC