VoIP
Voice calls can be compressed and packaged into voice-over-IP packets and sent over the public Internet or a more direct IP-based data link, thus by-passing the conventional telephone routes into a country.
A carrier receives calls in country A, turns them into IP packets using an IP gateway device and sends them over the Internet to another carrier or ISP in country B, which re-assembles the voice call and sends it out from a PBX. The cost to the carrier is the cost of a local call in the receiving country, not of an international call in the sending country.
Read more about this topic: International Telecommunications Routes