Any recommendations for an introductory book, review article, overview, or web site on IP telephony? -- not a highly technical coverage, but an overview of what's currently available, or likely to become available, for people considering moving, partly or completely, to IP phone.
The main things my page leaves out are Packetcable and the unbundled systems.
Packetcable is VoIP running on a dedicated subchannel on a cable system. The technology is VoIP, but the performance is more akin to a normal wired phone. The pricing tends to bundle in a gazillion features and flat rate calling, and to be somewhat but not hugely less than the local telco would charge for a similar bundle. They typically provide battery backup to make it somewhat resistant to power failure and real 911 service.
There's now some reasonable options for buying VoIP service piecemeal. A company in Germany called Betamax operates under a variety of names such as voipdiscount.com providing astoundingly cheap outbound VoIP service. Their usual rate for calls to the US is one eurocent/minute, but if you set up an account and put 10 euros into it, calls to most countries you'd want to call (US, Canada, landlines in most of Europe, etc.) are free for 120 days, then after that you use up your money (1000 minutes if you call the US), then when your money is spent down, probably six months since you put in the 10 euros, you put in another
10 euros and start over. They provide a downloadable softphone, but they are happy to let you use a VoIP phone or terminal adapter. I have an old Grandstream VoIP phone I bought on ebay for $20 plugged into my DSL router. They also offer local (not toll free) dialaround numbers in many countries, which is really great for those of us whose mobile service offers unlimited calls to local landlines, giving me the same mostly free international calling from my mobile as from my VoIP phone.
Incoming calls are a little trickier -- if you have an inbound VoIP number, Betamax will let you point it at your Betamax phone, but I don't know of anyone who will sell small numbers of incoming VoIP numbers. The largest vendor is Voxbone, who used to sell onesies but now has a 500 number minimum. A do it yourself setup like this also does not provide any voicemail or other features; for that you'd need to point it at a linux box running the Asterisk open source PBX or the like.
At this point I would not want a non-Packetcable VoIP phone to be my main phone, because they're still kind of flaky, but if you make a lot of calls, particularly international calls, they're a good deal.