I'm sorry, but whoever posted info about 911 being available after disconnecting land line is seriously wrong. Most cellular carriers do have access to 911 even w/o service. However, when you disconnect your phone service from your local carrier, you LOSE DIAL TONE. NO DIAL TONE = NO 911. Call your carrier and they will confirm this.
My boyfriend works for Cisco and has even admitted that VoIP products are great for recreational use but only a little better than Internet telephones. Some carriers who provide dial-tone through thier DSL lines, are NOT doing VoIP or Voice over DSL, but simply sharing the pair of wires going to the carrier which are more reliable but sometimes static even w/filters.
Another warning - those who subscribe to AT&T "digital" phone service also run risk of downtime during power outage. They provide small backup battery which would not last more than a couple of hours - so either invest in larger backup or UPS system if you're running these.
Finally - even w/my home office setup, I find it's cheaper (and more reliable) to buy the basic dial-tone service (AT&T non-digital) and use someone like
Airspring/Global Fibernet (or one of the many others on forum) for local toll and long distance which offers direct dial (no access codes) no prepay and quality line service for 3.5cents/minute anywhere in US.