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