kamailio/lib/srdb1/schema/kamailio-extensions.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE database PUBLIC "-//kamailio.org//DTD DBSchema V1.1//EN"
  "http://kamailio.org/pub/kamailio/dbschema/dtd/1.1/dbschema.dtd" [

  <!ENTITY % entities SYSTEM "entities.xml">
  %entities;
]>

<!-- Inclusion point for custom extensions for Kamailio -->

<database xmlns:xi="http://www.w3.org/2001/XInclude">
    <name>Extensions</name>
    <!-- Include extra definitions if found, it is not
         an error if they are not present, although it
         would produce a warning -->
    <xi:include href="extra.xml">
        <xi:fallback/>
    </xi:include>
    <!-- add more extensions here -->
    <!-- <xi:include href="more-extras.xml"> -->
</database>