You do not always have to change these settings but we have found that Draytek routers sometimes need a little tweaking for optimal VoIP usage. We recommend the following settings:
Firewall > General Setup:
Call Filter: Set this to Disabled
Data Filter: Set this to Disabled
Accept Large Incoming Fragmented UDP or ICMP packets: This needs tp be set to Enabled
Enable Strict Security Firewall: Set this to Disabled
Make sure you click 'OK' at the bottom of the page.
Finally, navigate to Firewall > DoS Defence: from the menu. Everything here will need to be disabled
SIP ALG also needs to be disabled and can only be done via Telnet. Thankfully this is already disabled by default on new Draytek routers.
If you do need to disable SIP ALG on a Draytek you must telnet into the router and run the following two commands:
sys sip_alg 0
The SIP ALG will now be disabled.