SIP ALG stands for Session Initiated Protocol Application Layer Gateway, and is common in many commercial routers. It intends to prevent some of the problems caused by router firewalls by inspecting VoIP traffic (packets) and if necessary modifying it. Sometimes the ALG can negatively affect SIP traffic so there may be a need to disable it.  

Draytek routers usually have SIP ALG disabled by default but that feature can be specifically enabled or disabled using the following methods.

Most version 3.x models

Telnet to the IP address of the router.

eg. On Windows, Mac OS X or Linux/BSD, open up a Command Prompt / Terminal window and enter:

telnet router_IP_address

For example: telnet

You will be prompted to provide username and/or password. These are the same as you use to access the router's web interface.

Run these commands to disable the ALG:

sys sip_alg 0

sys commit

The SIP ALG will now be disabled.

Run these commands to enable it again:

sys sip_alg 1

sys commit

The SIP ALG will now be enabled.

Firmware version 1.x models eg. Vigor2750, Vigor2130

The ALG is disabled by default. To enable it:

kmodule_ctl nf_conntrack_sip enable

kmodule_ctl nf_nat_sip enable

To disable it again:

kmodule_ctl nf_nat_sip disable

kmodule_ctl nf_conntrack_sip disable

Older Vigor2760 (firmware ver. 1.2.1 and above but below ver. 3.x)

Log in to the web interface and under Network > NAT, click the ALG tab then toggle the SIP ALG on or off as required.