UPDATE: the problem is caused by an unexpected change Draytek have made to QoS.
As is normal, we expect QoS to prioritise a class of traffic e.g. VOIP. That's how it allegedly worked in previous versions. In this version, they have decided to segment traffic instead of prioritising it. So instead of reserving a percentage of bandwidth for a class of traffic if it's needed during busy periods, they are permanently reserving the defined bandwidth making it unavailable for normal traffic. I had configured a good chunk for VOIP traffic so this would always get all the bandwidth it needed when it needed it, but that meant that bandwidth was no longer available for normal traffic regardless of whether VOIP needed it.
To me this is plain wrong. Some might want the option to segment traffic in this way and have dedicated bandwidth for some classes of traffic, but more usual is to just prioritise traffic when needed, leaving the full bandwidth available when there is no priority traffic.
As it stands, there appears to be no way around this other than to either accept a chunk of bandwidth is being permanently wasted or turn off QoS.
I'm still waiting from a response from Global support.
