Simply put, Asterisk is free software that even the most robust phone systems run on.  Countries and universities around the planet use Asterisk.  Two telephone service companies in Kalispell run their publicly available telephone service on an Asterisk system.  (We have worked on one; we know.)


It is open source, or free to use, just like the Android operating system.  This detail is disruptive to the telephone system industry, and we welcome how modern technology has become more accessible to more organizations.  Progress...


More details about Asterisk are explained in this article.