From a cost perspective, there are various taxes attached to a wire line bill that VOIP providers are not required to charge. Some of these taxes, depending on your monthly usage and plan, can be a few pennies. The "Federal Subscriber Line Charge" runs several dollars. (The tax on my phone bill is $6.38)
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. A VOIP provider, for the most part, has less overhead than a typical LEC or cable tv provider, so they can charge less for their services. If you live in an area where wires are strung from poles and wires are cut or a pole goes down, you won't be calling your VOIP provider, you'll be calling, via some other means, your Internet service provider.
But voice quality issues, E911 service issues, the 100% dependence on your IP and the reliance on your power provider add additional places where VOIP service can be lost.
It's true that VOIP providers offer alternative delivery of voice calls during service disruptions, (such as redirecting calls to a cell phone), but, since outages may not be readily apparent, you would have to leave your cell phone turned on 24/7 to be sure you receive all calls. Even then, if you go the cell phone as a back up, you may still miss a call or two, depending on you cell phone provider's quality of service in your particular location.
Maybe someday, VOIP's transmission quality will be enhanced enough to make it difficult to tell a VOIP call from a TDM call, but, in my humble opinion, that's still a ways off.