Hey there, thought I would throw my 2 cents in, I’m the author of the DSC Smartthings Panel app and devices, my current setup is outlined below, hopefully that helps complete the picture of how all this is connected.
DSC Alarm panel plus connected zones
-> Envisalink Module installed in panel
-> AlarmServer python app interfacing with Envisalink module + Smartthings
-> AlarmServer app sends REST API calls for each event sent to Smartthings App and translated to virtual devices to represent state of each DSC zone (motion, doors, whatever).
So far I don’t have 2-way communication built in, just one way from DSC Panel to Smartthings (this is mostly a security issue right now not a technical one just fyi).
You can’t send events back to the DSC Panel but you could Arm/Disarm/Panic the DSC Panel by sending a REST API call back to the AlarmServer app.
You can hook a siren/alarm up to Smartthings and trigger it for certain events but to my knowledge Smartthings doesn’t have a way to contact the police for you like a traditional alarm installed into your home.
With the DSC Panel that a few of us have and the Envisalink module you can have alerting for free (via text or email and also via Smartthings with integration apps) and alarm company monitoring (with police response) for cheap. Of course any traditional alarm panel will have monitoring/police contact options with various cost.
Hope that helps, let me know if you have more questions.
-Kent