Tmobile has a service coming out that I think would fit your needs.... Now I dont quite remember all the details but... There are two options (I Think) The first option has a specialized router manufactures by Linksys that accepts a SIM card and acts as both a wi-fi router and a VOIP access point. You would plug any standard telephone in to the router and you would have "land-line" service through the inserted SIM card in the router. The second option is to get a curve for a Pearl "II" (8120) which works with UMA. With a 9.99 add-on through tmobile. Any minutes you use while the phone is connected through a wi-fi hotspot, (Wether it be Tmobile Hotspot, Your home wi-fi router <Not necessarily the specialized linksys mentioned above> , or any wi-fi access point you will be not billed against your minutes) If you choose not to opt for the 9.99 add on plan. You will have excellent service anywhere you have a wi-fi connection, however you will be billed the appropriate minutes. (Mobile to Mobile, Nights And Weekends, Or your normal bucket of mintues) I would look in to it if your thinking about ditching the land line.
The above information is true to the best of my knowledge. How ever, I had some dental work done today and I've popped a vicodin and am drinking booze so YMMV. Have a wonderful day.