Question: how to connect fxo to fxs?
Answer: To connect analog phone lines to an IP phone system you need an FXO gateway. This allows you to connect the FXS port to the FXO port of the gateway, which then translates the analog phone line to a VoIP call. There are a number of different FXO gateways available. You can view a list of gateways that 3CX supports here.