Make sure this fits by entering your model number. S20 Advanced communication features to create effective business communication experience for small business, connecting to traditional lines, VoIP lines, and cellular networks SIP Ext max 20 Concurrent call max 10 Analog Ports: max 4 FXS/FXO ports BRI Ports: max S20 Advanced communication features to create effective business communication experience for small business, connecting to traditional lines, VoIP lines, and cellular networks. SIP Ext max. 20 Concurrent call max. 10 Analog Ports: max. 4 FXS/FXO ports BRI Ports: max. 4 BRI ports GSM Ports max. 1 GSM port Storage Build-in TF Card Expand Slot.