The Draytek 2700V was one of the dodgy routers.

For some reason it would reboot sometimes on an incoming VoIP call, losing internet and VoIP connectivity for a couple of minutes. Not ideal.

I could 100% reproduce the error by calling out on one of the VoIP lines and back in through the other. Reboot every time.

Draytek technical support never did get to the bottom of it (i.e. whether it was a hardware fault or a design flaw). Draytek tech support has no VoIP equipment to try or phones to call out on.

So it has gone back for a refund under the Sale of Goods Act 1979.