I've found this about QoS & VoIP:

https://www.draytek.co.uk/archive/kb/kb_qos.html

But many of the screenshots don't match the Vigor2860. Specifically, where are the settings to control overall bandwidth:

https://www.dropbox.com/s/v4gnnmhtlup0hgp/VOIP-3.png?dl=0

Answering my own question but I suspect because the later versions of the firmware have speed measurement routines baked in, you don't have to manually specify the bandwidth, the router works it out for itself. So if you've set 25% for class 1 and the link is currently running at 10Mbps, it'll get 2.5Mbps. If the link gets faster to 20Mbps, then it'll get 5Mbps.