The CooVox series EX16S is an FXS port expansion box especially designed for ZYCOO CooVox series IPPBX system. Each box provides 16 FXS channels for connecting analog phones and fax machines to your CooVox V2 IPPBX system as extensions through your LAN. Each FXS port uses an individual SIP extension number to register to the CooVox V2 IPPBX system just as an IP phone does.