Seems my Sky plus box isn't playing nice with Voip. My Sipura SPA 3000 reports line voltage -5V, ie "phone receiver lifted". I've unplugged the Sky plus box (contract requires it be plugged in at all times), so that I can use my phone line.

Update: 18:00, back from work, plugged it back in, all working fine. Sky lets me have my phone line back!

I was always a little uncertain about signing up for Sky, only because of the "constantly connected to a phone line" clause. Not, strangely, for privacy reasons - Murdoch can know which episodes of "Charmed" I've watched - but for the practicalities.