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