To disable SIP ALG on a Draytek router you will need to Telnet into the router and unbind SIP from port 5060. If you have a Telnet Client installed, you simply need to run the following commands in the command prompt, which can be accessed by pressing Windows Key + R, then typing cmd, if using Windows. If you are using Mac OS X or Linux, simply open a terminal window.
telnet [Router IP Address]
sys sip_alg 0
NB: Enter your router's IP address in place of [Router IP Address].