It is a method of communication based on text messaging between two fax machines, the first one calling to other one to send a fax, the recipient receives a text on A4 paper and after that communication process ends.
Main portion of the advantages of the Asterisk server appears from it being an open source downloadable product. Most of the users, especially business people, can find it difficult to understand the idea behind this principle, or free software, but a business models behind open source are different and specific, as in case with Asterisk server.