S100 is a cost-effective business communication system with a base configuration of 100 users and 30 concurrent calls and can scale up to 200 users and 60 concurrent calls. With Yeastar S-Series, all the telephony interfaces are modular and customizable, S100 allows 2 Expansion Boards to customize FXS, FXO, ISDN BRI, GSM/3G/4G, or E1/T1/PRI port.