I switched my telephony over to VOIP earlier this year. I signed up for an account with Irish VOIP co. Blueface and the service has been, on the whole, fairly reliable.
In fact, I was so happy with the service I recommended it in a Sunday Times article on VOIP this last weekend.
For now, if you need to call me, call me to my mobile (+353 86 384 0828).
Update: Phone service has returned to normal after nearly 24 hours and no explanation of the downtime.