Speaking about VOIP nowadays is speaking about Skype.
Indeed there is a possibility to use a cordless phone. Even without giving up the landline.
Check out this:
http://dualphone.net/Hete you can find VoIP phones. I use a dualphone 3088. The base station is connected with a cable to my router, the phone is wifi connected to the basestation. It works like a charm.
I can call Skype or landline, as i choose. You can also use it without a landline connection, you can get a skype phonenumber for incoming calls.
I have a Skypeaccount, which costs me a couple of bucks each month. I can call all landlines in Europe for a fraction of the cost a landline call does cost me.
