A mystery carrier in the UK will soon be partnering with Challenger Mobile to offer free VOIP from normal phones without the annoying necessity for a Wi-Fi network. The prospect would allow a mobile phone user to call any country in the world for completely free if the other party supports the techology. So how does it work?
It's a SIP (Session Initiation Protocol) based service, meaning that it's compatible with most cellphone service standards already. When I call you through SIP, your number would be converted to an email-like extension automatically and be sent to a central server. That server would then contact us both to establish the connection. And in the case that your service isn't SIP supported, calls can be appropriately routed to standard telephone networks.