Automatic NAT Rules

A lot of our customers do Voip using a combined WLAN/Voip-Box.
Our cpe run a nat configuration.
The sip-helper does not seem to work.

So we need to do a static DHCP-Entry for the voip-box
and an dest nat-entry to pass several UDP-Ports to the box.

Does anybody know a solution to do this automaticaly?