http://blog.o2.co.uk/home/2011/11/o2...ts-simple.html

What is O2 Connect?
Ever heard of VoIP? It stands for Voice over Internet Protocol, and it simply means making calls using an internet connection rather than a traditional mobile phone network, like how Skype works on computers.

O2 Connect works in just the same way, but on your mobile. On a Wi-Fi-enabled smartphone, it simply routes calls and texts to any UK landline or mobile number through the office or home network you’re connected to - handy if you’re buried deep inside a building with weak reception.