It makes your phone vibrate when outgoing call is answered, a feature missing in stock dialer app of most phones (and Google Dialer too).
This decorator only works for AOSP-derived dialer (with app ID "com.android.dialer") and Google Dialer app. It works by reading the sticky notification shown by dialer app when phone call is ongoing.
If this mechanism is confirmed to work for any other dialer app, feel free to send a pull-request.