If you use a service like Vonage then…

1. You lose reliability of the phone system.
2. You sacrifice quality.
3. You lose 911 service and most importantly…
4. Update: You can use only one phone plugged into the ATA, unlike PSTN where you can jack your handset in pretty much any phone outlet inside the house. (Of course, you can buy a wifi handset for about $100, but that’s not the same thing as what I am mentioning.) I did find a way to get multiple phone-jacks working on a single VoIP line, thanks to a AT&T Callvantage brochure. Still it doesn’t address the ease of use issue. I know smart folks like those in the comments section, can do it easily, but can mom?