This article explains how to make “free” international calls and lists providers of these services.
APCmag explains: “To use these services, people dial a local PSTN or mobile number from their phone, and then, once connected, dial in the number they want to call overseas, and the VoIP provider connects them.” The providers earn revenue from mobile termination rates (“the fees mobile phone companies charge other carriers to terminate calls on their networks”; also known as incoming termination fees), while the operational expense of providing the international call is largely negligible due to the use of VoIP.
While the international call is free, this option is cost-effective only if the user can call the provider either cheaply or freely. If the provider's number carries the 04 prefix, the user must therefore be able to call mobile phone numbers cheaply or freely - typically through capped mobile phone plans or plans offering untimed calls to mobiles (e.g. PennyTel's Talk Till You Drop) - in order to take advantage of this option.