Draytek Router VoIP Configuration

How to configure Draytek 27xx and 28xx series routers for VoIP

This config guide applies to following routers:

  1. Draytek Vigor 2820
  2. Draytek Vigor 2830

Settings to change:

  1. Disable SIP ALG
  2. Allow traffic to/from the following IP addresses: and (Server IPv4 ranges), may also be ranges, and
  3. Update Firmware to latest version
  4. Untick “Accept large fragmented UDP packets” under the firewall configuration

Issues such as one way audio, lack of incoming calls, registration issues and etc. can be due to SIP ALG

To disable SIP ALG you have to telnet into the router and enter the following commands.

 1. > sys sip_alg 0 -- Disables sip alg
 2. > sys commit -- Apply changes
 3. > sys reboot -- Reboot router

Once the router is back online, reboot the IP phone or press re-register.

Incoming Call Problems

This is caused by a large IP SIP packet that is fragmented, but the router will not forward it. I understand that the new version will work if the checkbox under firewall -> general setup -> Accept large incoming fragmented UDP or ICMP packets is UNTICKED. This is the opposite to what you would expect.