...
Native support for ISN signaling has been added to sipXecs as a redirector plugin. The ISN signaling capability can easily be enabled using the sipXecs Configuration Server by selecting the respective check box (go to the Registrar service under System / Servers).
...
Outgoing ISN dialing is enabled using the sipXecs configuration interface. Go to System / Servers and then select the Registrar service and select the ISN checkbox. Optionally a prefix can be selected that needs to be dialed but will be stripped before the ISN lookup takes place.
The sipXecs ISN redirector plugin performs a native ISN lookup. If you control the DNS server for your domain request that freenum.org delegates the zone for your subdomain <your ITAD number>.freenum.org to you so that you gain full control over ISN mappings.
...