Zyxel 660 HW SIP incompatibility

Zyxel have included a Application Layer Gateway in the 660H series adsl modem/routers.  Unfortunately this SIP-ALG breaks  SIP Voice Over IP!

Even more unfortunately you cannot turn it off from the web interface.

 The fix is to logon to the device via telnet and enter the following command:

ip nat service sip active 0

You will receive the response:

SIP ALG Disabled

Reboot your modem/router and the SIP device, you should now be able to make and receive calls.