DrayTek
DrayTek Various User Guides for DrayTek routers.
DrayTek Vigor2825 - User Guide
image coming soon 150 DrayTek Vigor2825 - User Guide
DrayTek Vigor2860 - User Guide
image coming soon 150 DrayTek Vigor2860 - User Guide
DrayTek Vigor2830 - User Guide
image coming soon 150 DrayTek Vigor2830 - User Guide
DrayTek Vigor2820 - User Guide
image coming soon 150 DrayTek Vigor2820 - User Guide
DrayTek Vigor 2820/30 - Disable SIP_ALG
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If you are experieincing issue with VoIP such as calls cutting off after a set amount of time, one-way voice, or issues transferring calls you may need to disable SIP_ALG on the DrayTek router.  This will ensure the data sent to the handsets/PBX are not malformed.  

This guide shows how to turn SIP_ALG on or off.

DrayTek Vigor 2820/30 - Downgrade Firmware (VoIP Issues)
image coming soon 150 If you are having issues using VoIP whilst using the DrayTek Vigor 2820 or 2830 you may need to downgrade the firmware. There is a known issue with this model in regards to registration attempts failing. This guide shows where to download the firmware, which version you require, and how to apply it to your DrayTek router.
Hardware available from Amazon