Camp-On for ext line

Hi to all,
I am new in the forum and I am from Italy.
I have used asterisk without GUI in my PBX installations, but after I tried Freepbx I think I will use it forever.

I have developed in my asterisk pbx something like Camp-on, when I dial a number (internal extensions or public) and it is busy asterisk will try itself to call the number for 30 min every 20 sec the busy number and when it find the number available will call me at my extensions.

Camp-on make the same but, as I read an the forum, only with internal extensions. Is it right ? Or the module can call the external numbers too ?