In order for incoming calls to arrive to your VoIP devices, your firewall must support NAT so that phones with internet network addresses can be reached by the FracTEL system.  In addition, your firewall should allow listening on TCP and UDP ports so that inbound traffic can reach the VoIP devices. 

We recommend using NAT instead of opening up specific TCP and UDP ports or ranges.  This allows your firewall / router to be as secure as possible.