How to send VoIP notification from one user to an other when video call is made

Im trying to send voip notification so that on other end user can get call notification like any other video calling app. I used pushkit for this purpose but issue is when i send push from pusher app giving token received from pushkit, call is initiated while i want to send this push notification for voip from my app. So that when user taps on call , the second user can get call on screen.
How to send VoIP notification from one user to an other when video call is made
 
 
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