Merge beetwin 2 freepbx voip centers

hello there …
how can i merge between 2 freebpx centers that the connection between them is over the Net? the 2 domains see echothere. and work prefect but what i need to do in side the freepbx to configure them for from one office you can only put the extension and it will automatically go to the extension that it’s needed .

thanks ,
oren