I've been using voice over the public Internet for a long time, and the only times it has been unavailable (at a time that I tried to use it, and hence noticed) has been when my DSL has been down. When my DSL has been down, by and large, my analogue Bell Canada line has also been down.
just last eve, we noticed that voip from our hawi line was dead, allison did not answer our hawi phone. investigation (dialing the fax number:-) made us suspect that all phone lines were out. but users complained that voip was the problem! they did not seem happy when i said that, considering it was verizon phone lines out (both lines, voip and fax), it would still have been dead without the voip kit. verizon fixed the lines this morning at 06:30 hst.
Fortunately, probably like everybody else here (and, increasingly, most people within the likely demographic to which VoIP service is marketed) I have a cellphone. The next time someone melodramatically collapses in my living room clutching their chest and mouthing "call an ambulance" I suspect we will be ok.
i also have the voip adapters' dialplans (that's bellhead for configurations) set so 911 and 411 short-circuit directly to the local pstn. this lets the blame fall appropriately, and also means that 411 will get local directory assistance, not the one from nyc. my son, a luddite, got rid of his pstn voice and took his ip provider's voip service. he did the install using their csr support, and even got his 802.11 network back up. so it can't be all that bad. a few years' experience, from my very small view of the world, is that voip is about as reliable as pstn, except o be careful of layering, i.e. pstn-voip-pstn etc. adds the unreliabilities o it was all designed by bellheads, so it is disgusting to configure but o it can be really cool, like being able to make essentially free calls from my laptop in very strange places in the world o it sure lowers the costs, e.g. six cents a minute to china without even hunting for prices so, i am sure it does not meet everyone's needs, seems poor quality to some, ... but it's deploying at least a decimal order of magnitude faster than ipv6. so, rather than pretend it sucks so badly it can be ignored, i suggest we work on what it needs to be better and to scale really well. randy