The companies revolutionising how we make mobile calls
Using apps to make internet calls has been available for a while, but the caller and receiver often have to have the app installed and a data signal is needed for it to work.
But a company called onoff is changing this. It uses the GSM network as well as data to connect to the handset, allowing SMS text messaging, producing a wider coverage and only one person, even if it is the receiver, needs the app.
Customers can also be issued with multiple numbers and adopt a number that is local to the country they are in.
Meanwhile, a company called FreedomPop is offering a free Sim card with free calls and data.
BBC Click's Dan Simmons finds out more.