This is not a big problem. Lots of services today offer ringing multiple phones at once. I use it myself all the time. Call my number and it rings 5 different phones (but not my cell phone because that goes to voice mail immediately if off.) Not a problem. Whoever is first wins, or if the cell company wants to, it could just have all answerers be put in a conference call and bill lots of airtime.

I just don't see how this is marginally useful? I guess you never forget your phone. If you don't, you wouldn't see the value of this, but for those who sometimes go out without their phone, or even those who leave one phone downstairs in a charger while they are upstairs, we can see the value and it's high.

High enough that they could even charge more for it, ie. if I want to use the backup phone in my car they could bill me per minute on top of my pool of minutes, and let me decide if I want to answer or make calls from it. But I don't think they need to go that far to make this a revenue generating service, and some users would decline to use it if it came on those terms.

I think with this available lots of people would put a cell phone permanently in their car, either off or under the seat wired in to car power. Then, if they went out without their phone, they would be able to fix that.