It uses the same open GSM audio codec implementation as John Walker's Speak Freely (http://www.speakfreely.org/history.html http://www.speakfreely.org/history.h

It uses the same open GSM audio codec implementation as John Walker's Speak Freely (http://www.speakfreely.org/history.html http://www.speakfreely.org/history.html, https://web.archive.org/web/19970511165210/http://www.fourmilab.ch/speakfree/windows/ https://web.archive.org/web/19970511165210/http://www.fourmi...) from around the same time. https://imgur.com/a/y4De3Wc https://imgur.com/a/y4De3Wc