Additional Details ------------------------------ Product description: Model: YST-D30 Yeastar YST-D30 DSP Expansion Module for S100 and S300 The Yeastar D30 is a DSP module, used to expand the capacity of S-Series PBX. For each D30 module added, the max amount of users increased by 100, and concurrent calls increase by 30. Use up to 1 module on the S100, and up to 2 modules on the S300.