This is not really new technology, there are two paths in history it maps to
One - Rabbit - Use your rabbit phone around the house (it was a DECT cordless) or at a rabbit point.
http://news.bbc.co.uk/1/hi/technology/2175804.stm
Two - BT did something like this about 5 years ago, didn't take off then either, probably the billing now is more seamless but still much the same product.
O2 seem to have been offering a similar product in Germany for about a decade
http://shop2.o2online.de/o2/interess...pageframe.html
Here it is before fusion it used to be onephone
http://www.bt.com/userguides/downloads/btonephone.pdf
http://www.findarticles.com/p/articl...17/ai_19605043
It has the problem of only one mobile, also that mobile tech is fashion driven so in 12 months time the mobile will be due an upgrade and cannot be as it is part of the fusion thingy. Also lose the mobile and your home cordless is lost too.