This hybrid S-Series IP-PBX is rack mountable. Besides SIP-trunks and SIP-extensions it can be also modularly extended with BRI ports (point-to-point and multipoint interface) and with an analog port for the public telephone network (PSTN). Apart from this the optional upgrading can build analog extensions as well as GSM, 3G/UMTS and 4G/LTE functionalities. All PBXs provide one audio input and one audio output. Yeastar S20 is a PBX for up to 20 extensions. It allows 10 simultaneous calls. With Yeastar S-Series, all the telephony interfaces are modular and customizable, whether it's FXS, FXO, ISDN BRI, GSM/3G, or E1/T1/PRI. S20 has 2 onboard module slots, S50 and S412 supports 4. S100 and S300 require the combination of EX08 Expansion Board and Telephony Module to add telephony interfaces. S100 allows 2 EX08 Expansion Boards and S300 supports 3. Compatible with IP phones Fanvil, Polycom, Snom, Yealink, Vtech as well as further popular VoIP-terminals Specifications 20 Users 10 Concurrent Calls Up to 4 FXS/FXO/BRI Ports Up to 1 GSM/CDMA/3G/4G Channels VoIP Trunks: 4