A short foreword:

Many installed PBX’s are capable of H.323, but it will be difficult to upgrade them to understand SIP. So a challenge to interconnect sipx with existing PBX via H.323 protocol appears. Below is an example implementation via freeswitch/mod_h323.

The principal idea (Thanks Josh) of how to do it documented here: http://wiki.sipfoundry.org/display/sipXecs/Custom+FreeSWITCH+programming

The goal:

The goal is to make the following scheme to work.

sipX  =====sip=====  FS  =====h323=====  PBX

Steps:

  1. Compile FS with mod_h323 on the sipx server.
  2. Create separate FS sofia profile and dial plan.
  3. Configure sipx dialplan.

Step 1. Compile FS with mod_h323.

The main idea of recompiling FS on the sipx server is documented here:
http://wiki.sipfoundry.org/display/sipXecs/HowTo+recompile+Freeswitch+on+a+CentOS+5+sipXecs+install

The difference is that before actually compiling FS with mod_h323 one should compile prerequisite libraries (ptlib, and h323plus). It is documented here: http://wiki.freeswitch.org/wiki/Mod_h323

So the whole sequence will be:

Now you should be able to stop old and then start new FS instance by

sipxproc