Over the last few weeks we have been taking lots of support calls from people with NAT issues with phones using Draytek routers.
The 2820 seems to be the main router affected but it could be others.
The problem manifests itself with SIP phones losing Registration to the SIP server with 408 time-out messages. In most cases it is weird in that some phones on the network will work fine and others wont.
The fix (which is confirmed by Draytek themselves to a couple of my customers) is to downgrade to firmware version “333”.
This problem occurs even if you have the SIP ALG turned off. Please ensure the ALG is off as this can cause even more problems, it is usually off by default these days.
As a side note, please make sure you are not using a Draytek router with “voip” ports, the model number will usually have a “v” in it. These are not suitable for use with stand-alone SIP phones connected to the network, they are only use if you are ONLY using the built-in VoIP ports. There is no fix other than swapping the router out as far as I’m aware.