The Vibration Call Alert function only works with the combinations of the rotary encoder buttons A/B and Reply/Answerback button.
To Send call signal to other users:
Push the rotary encoder button A or B and push the Answerback/reply button to form a combo key that when pressed will call whichever destination has been selected on the beltpack. The call receiver will alerted by a call signal or the beltpack vibration or both depending on the alert options selected at the receiving end.
To receive the Call signal & vibration : ensure the option is selected in the Alarm menu of your BP . Make sure Vibra-Call  mode is enabled