Standard Tlephonique ip 2 FXO Easy to use. Powerful in Production Built for Small Business Yeastar S20 is a stand-alone PBX system with unrivalled range of features for business with less than 20 users. It’s built to be simple to use and powerful in production. 20 Users 10 Concurrent Calls Up to 4 FXS/FXO/BRI Ports Up to 1 GSM/3G/4G Channel 20 VoIP Trunks