analog adapter

We're migrating from a Cisco phone system to Zoom.  We currently have a door access system with an external doorbell/speaker that connects to a Valcom door answering system.  Two wires come out of the Valcom into a Cisco router's FXS port.  The router detects an off-hook condition when the doorbell is pushed and dials a number on the Cisco system.  I'm trying to find a solution that replaces the Cisco system.  As I see it, we need to be able to land a two-wire connection on it, have it monitor for when the line goes off-hook, and then dial a pre-programmed number.  

Any ideas on how to do this?