VoIP technology (voice over IP) allows you to transmit a signal over long distances through the VoIP servers. A standard PC with installed software PBX (e.g., Asterisk) can serve as a server. Through SIP communication protocol, it is possible to redirect international calls through a number of intermediaries in the country where the user is located. They use a special device that converts a VoIP to GSM called a VoIP GSM gateway.