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.