[1] IDT Corp used a hardware solution that was never widely accepted and ultimately failed and was surpassed by a software solution concurrently developed by Jorge Blanco at MCI Telecommunications for The United Nations Development Programme and their staff Wolfgang Scholtes and Kumar Navaratnum.
This may mean dialling in the North American format, as most callback services are US-based, and callers are effectively calling from the US.
People who provide or use callback services have argued that these telephone companies are often state monopolies that charge unjustifiably high prices for international calls.
Other countries have sought to use technical means to prevent callback, by blocking the inbound and outbound calls made to and from DID numbers, or by disabling touch tone dialing.
International callback has been credited with opening global telecom markets because it enabled competition to start up even if regulatory restrictions existed.