All cordless is fine; assuming you have enough UPS systems in your house. Most people have an UPS for the PC and gear, but what good are those phones if you have a power outage?

As USA2K mentioned, do a search on disconnecting Ma' Bell's main incoming line from your phone jacks. It's a very simple process. "Assuming you aren't using DSL, which requires keeping the jack that the DSL modem plugs into, connected to the internet provider". For the person who only used Ma'Bell for phone service, it takes about 3 minutes to disconnect Ma'Bell. If you have a newer Ma'Bell NI (Network Interface), then you simply need to just unplug a phone cord in the box and leave all the wiring alone. That takes about 8 seconds. Once you do this, you simply plug the voip adapter into one of your phone jacks, and ALL of your phone jacks work just like a normal phone system.

But do some searching for something with pictures or Youtube video so you know exactly what you're doing. If you don't do it correctly, then the Ma'Bell line will have voltage on it; plus your voip adapter will be putting out voltage. Not good for the adapter. And even if the Ma'Bell didn't have voltage on it, if it was still connected to your house wiring, there would probably be a few MILES of wire connected and that wouldn't allow your phones to ring most likely.

Here's a couple of links. Caution: If you don't feel comfortable reading this material, then you really have no business playing with it. It's not complicated, but there's a reason some of us get paid a lot of money. If you feel comfortable reading this, it's easy. If you don't feel comfortable, buy one of the cordless units with 4+ cordless handset.

http://www.voipmyhouse.com/
http://www.ehow.com/how_5997259_add-...hone-line.html
http://www.ehow.com/how_5701936_wire...ouse-voip.html
http://www.engadget.com/2006/06/20/h...r-phone-jacks/