@book { VoiceoverIPunterLinuxNebenstellenanlagemitAsterisk,
    title = "Voice over IP unter Linux: Nebenstellenanlage mit Asterisk",
    author = "Michael {Sauer}",
    year = "2013",
    publisher = "GRIN Verlag",
    address = "München, Germany",
    doi = "10.3239/9783656445357",
    url = "https://www.hausarbeiten.de/document/229440"
}