You will need to log into the Web Interface for your Zyxel Zywall device.
Once you log in click on ALG on the side tab:
Uncheck Enable SIP ALG
Change the SIP Media Inactivity timeout to 120 seconds.
Click Apply.
You will also need to change the UDP time out as well under Firewall --> Session Control.
On the left side click Firewall
Click the tab Session Control
Change the UDP Session timeout to 120 seconds
