forwared site to second local server

Hi Team,
I wan to access my 2nd server web control panel (running freePBX) from out side of private network. Currently my 1st PC is running ISPConfig and 2nd one PBX.

Is there any way I can create a site in ISPConfig and forward that url to my 2nd PC.

Here is the scenario:

http://example.com (a site in ISPConfig) -> http://192.168.1.50/ (PBX)

I was trying to use Proxy with the following line on Vhosts_ispconfig.conf:

<VirtualHost xxxx.xxx.xxx.xxx:80>
ProxyPreserveHost On
ProxyPass / http://192.168.1.50
ProxyPassReverse / http://192.168.1.50/
ServerName www.example.com
</VirtualHost>

That did not work; apache2 restart did not recognize ProxyPreserveHost.

Thanks in advance.