DrayTek routers have advanced Quality-of-Service [QoS] and Voice over IP features designed to simplify setup and operation with VoIP traffic.
Enable SIP Application Layer Gateway
The SIP ALG feature removes the requirement to open large port ranges on the firewall by examining SIP setup packets to discover the network ports which are selected for the encapsulated audio stream. The selected ports are then automatically configured for traffic to flow and closed upon completion of the call.
The ALG feature is in the NAT menu :
Check the Enable ALG box and then the check box for the SIP protocol. Ensure you enter the session setup port (5060 is the default for SIP traffic) and the protocol used. Most VoIP systems use UDP as default, but increasingly TCP is used, particularly when voice encryption is enabled.