The problem I see you have Patrizia is cost, certainly in the UK. The cost of calling both for mobile and fixed has come down dramatically. Your £200 investment in the devise and base station would take nearly 2 year to recoup. I base this on my wifes pay as you go mobile (no monthly charge, only call charges) assuming call cost 10ppm and an average of 20mins per week. If you assume some of calls replace fixed calling the situation is worse (national calls for 2ppm and to the USA for 2.5ppm).
The value of the proposition is to replace expensive WiFi access (T-mobile in Starbuck is £5ph).
The voice calling would add value to the proposition. You should talk about integrated communications, not voice and data.
Also worth noting is Skype is planning to enable WiFi capability on its peer2peer voice platform.
Not saying it is not a bad idea, just some of the challenges I see.