SIP ALG (Application Layer Gateway) is a feature which may be enabled by default on most DrayTek Vigor routers and inspects VoIP traffic as it passes through and modifies the messages on-the-fly.
In few situations this is useful, but in most situations SIP ALG can cause problems using the service. Very often, SIP ALG is poorly implemented causing VoIP messages to be modified in a harmful way or simply corrupts them. In such cases you may experience issues registering your handset, one-way or no audio, unable to receive calls, and issues with Busy Lamp Field (BLF) and Message Waiting Indicators (MWI).
For our Hosted VoIP service, including out Single User services, SIP ALG must be disabled on your DrayTek Vigor router.
This guide explains how to to disable SIP ALG on the following DrayTek models which are running firmware versions older than 3.8.5:
Even if your particular DrayTek model is not listed, these instructions should still apply if your model is in the same series.
If your DrayTek Vigor router is running firmware version 3.8.5 or newer then please refer to the following guide:
telnet 192.168.1.1
and hit Enter
Note Recent versions of Microsoft Windows may not have Telnet installed by default.
To install Telnet, click on Start → Control Panel → Programs → Programs and Features → Turn Windows Features on or off → Ensure TELNET CLIENT is checked and click OK.
sys sip_alg 0
and hit Enter
sys commit
and hit Enter to save the newly applied change
SIP ALG on DrayTek Routers at at www.draytek.co.uk (registration required to see article).
For information on setting up Quality of Service (QoS) see this guide.
If you are running firmware version 3.8.5 or newer then please refer to How to Disable SIP ALG on the DrayTek Vigor Series Routers (firmware 3.8.5 and newer)