http://wiki.freeswitch.org/wiki/Sofia_Configuration_Files#User_Agent_.5Buser-agent-string.5D

As per the above link, we can change the User Agent String, but I added this param name but does not seem to work.

[user@freeswitch autoload_configs]$ vi sofia.conf.xml

<configuration name="sofia.conf" description="sofia Endpoint">

  <global_settings>

    <param name="log-level" value="0"/>

    <!-- <param name="auto-restart" value="false"/> -->

    <param name="debug-presence" value="0"/>

<param name="user-agent-string" value="Test Server"/>

  </global_settings>

  <!--

      The rabbit hole goes deep.  This includes all the

      profiles in the sip_profiles directory that is up

      one level from this directory.

  -->

  <profiles>

    <X-PRE-PROCESS cmd="include" data="../sip_profiles/*.xml"/>

  </profiles>

</configuration>