01760015800, 01717583201 Yeastar S20 VoIP PBX business phone system supports 20 users, 10 concurrent calls, up to 1 GSM / 3G / 4G channel, up to 4 FXS / FXO / BRI ports, freescale ARM A9 industrial grade quad-core CPU processors, protected from internet hackers and viruses with the built-in firewall, TLS, SRTP and more.