I rigged up one of these so my wife could go push the button and her phone would play an alarm sound so she could find it when it goes missing.
Was a good hack for a bit, but then the children figured out they could actually use the same button to _find their mom_, since the she was usually colocated with the phone!
Did you need to write an app for the phone?
I had the script call the phone, and set the phone to "emergency bypass" for that incoming number and set the ringtone to alarm bells