I guess your ask is somewhat generic , so I can answer generically .
I believe if I had this as a challenge I would look at:
Can the motion or whatever detector trigger a script via something ? I’d drop a file into a spool for Asterisk to place an outbound call.
I’d have the call setup to call a bridge address on the MCU.
In my limited experience , there are devices like security cameras and such which can trigger an event which can result in a SIP call, but they wouldn’t necessarily help you here.
To extend the musings further, you could use the XML API with the phone to trigger the phone the call the MCU instead of using asterisk by posting to the phone’s /CGI/Execute endpoint with a Dial URI.
