This read this (source)

“A California, US-based company — GrandCentral — has just turned imagination into reality, says a New York Times report.

The company allows you to choose one, new unified phone number which sort of ‘replaces’ all your other numbers: mobile and landline. It’s a new Web application that lets you consolidate all of your phone numbers into one number.
Once you get this number, all you need to do is send it to everyone you want to. Whenever somebody dials your new GrandCentral phone number, all your phones will ring simultaneously, and you can pick up the call on any of your phones.

No longer will anyone have to track you down by dialing each of your numbers. It will no longer matter if you’re home, at work or on elsewhere. The GrandCentral phone number will find you.”
Definitely, there is a big potential market for “Unified Phone Number (UPN)” and also there are lot of room for expansion.

Visit http://www.grandcentral.com and get your UPN.”

The only problem I see here is when you live with a friend or with a boy-/girlfriend. What if I want to reach person A, who lives with someone, and he’s not in his house. Then the house phone will still ring, that someone will want to pick up the phone as well (or just might be the closest to the phone and so, first to answer) and I still don’t reach the person I’m looking for. So then ‘that someone’ gives me what…? The mobile number, which is in fact absorbed by the GrandCentral number, so the fixed phone rings again, but ‘that someone’ can’t pick up because he/she knows it’s for person A. Hmmm.

This is a good solution if you live alone. Not if you share a house/phone.