ZyXEL

From the WWW interface choose 'Advanced' and 'ALG'. Uncheck the 'Enable SIP ALG' box.

**For Older versions of the sofware**
From the command line you must turn off the SIP ALG:

Telnet into the router. Choose (24) System Maintenance and (8) Command Interpreter Mode. From the command line type:

ip alg disable ALG_SIP

You should receive the message "SIP ALG Disable"

You will then need to reboot the router for the change to take effect. Reboot the router and then the phones.