Note that, as this process may temporarily interrupt your internet connection and phone service, we recommend doing this in an off-peak period or outside of business hours.
Learn more about SIP ALG in our knowledge base article here: SIP ALG
How to Disable SIP ALG on Draytek Routers
Login to your router's web interface and select NAT > ALG from the menu on the left.
Uncheck Enable ALG, then select OK to confirm your changes. If prompted, restart your router to finalise the change.