Vonage VOIP has sorted the 999 (911) issue as something in their set-up fixes your geographical setting and does the appropriate thing with these calls.
I guess you might cock it up if you take your VOIP hardware and plug it into someone else's router. What surprised me is that they were prepared to offer me any dialling code area I want so I could appear to run my business from Edinburgh (or even somewhere outside of the UK) even though I am, in fact, in Berkshire. Sneaky or what?
Rans6....