NOR charts thread, page-2391

I suspect its a call back type service, ie: it establishes a call to both parties (possibly initiated by an app, or perhaps you dial a set freecall number then enter the one you want to connect to) and it routes it cheaply between the parties.

ANI callback is another way some providers achieve the same thing - it works well, basically VOIP without a SIP client or configuration. Downside is that someone pays for the PSTN termination which in some countries can be quite high.

Not 100% sure that's how rebtel is doing it, but I don't know of any other technique. Will have a look further as it is an interesting proposition.