Modified September 2015
Zyxel Zywall USG50 - newer firmware:
Turn OFF Enable SIP ALG.
Turn ON Enable SIP Transformations.
Turn OFF Enable Configure SIP Inactivity Timeout.
Enter Pass Rule for All OnSIP IP Addresses
Increase UDP Timeout from 25 to 300 under Firewall tab, Session Control
**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.