Yeastar S20 is a modular VoIP PBX for small-to-midsize businesses that lets you choose what kinds of port work for your deployment. The S20 has two on-board module slots and can support the following modules: S2 Module: 2 FXS (analog) O2 Module: 2 FXO (analog) SO Module: 1 FXS 1Fmore...