Just thought I'd quickly contribute to this thread... looking at the vonage service and have contacted their support with some questions.. sure other people have the same questions --
They state on their FAX page that they provide a free dedicated fax line with the service. This, according to their customer service, is only partitially true. They provide a free dedicated fax number and line for business accounts. It's $9.99/month on residential accounts for 250 minutes of ourgoing fax calls (still a pretty damn good deal) and any normal fax machine or fax modem works fine with it.
Also, being a past user of SIP based voip software - I've had a real problem getting it to work though a FreeBSD firewall. They never clearly stated it would work, but they did say it will work with any NAT based firewall/router (which is what I use on my BSD box)... so I'm probably going to cross my fingers and hope for the best on that
As for the power outage thing... you can get a cheap UPS for under $100 now and that should power the router and voip gateway for quite a few hours... probably isn't going to save your phone service if the power is out for days, but you'll get a few hours out of it at least.
Anyone else have any experiences they want to share with this before I spend money I shouldn't spend on more communication services? lol
CGI