How are they listening to a few minutes before the button? A "temporary" recording of the first 2 minutes of every call? I feel like there would be some privacy concerns.
It's a common feature in better dashcams--you get say the 30 seconds before the thump that triggered it. Likewise, many high speed cameras that record some short action. They're actually always running, but dumping the end of the data, they only "record" when the trigger happens.