In order to turn off the SIP ALG on Zyxel P 600 Series Routers you will need to telnet on to your router
1. Open a Command Prompt, cmd


2. Type telnet, then the IP address of your router e.g. "telnet", then press Enter
3. Then enter the admin password for your router and press enter, the default for these routers is 1234
4. Then in the menu option enter "24" and press Enter
5. Then enter "8" and press Enter
6. Then enter "ip nat service sip active 0" and press Enter

The SIP ALG should now be disabled