How can a notification service extension receive a VoIP push? (as opposed to receiving a non-VoIP push)

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If a voip push (i.e. a push with a topic of com.company.app.voip as the push topic) is sent to a handset then it is delivered directly to the app via:

pushRegistry:(PKPushRegistry *)registry didReceiveIncomingPushWithPayload:(PKPushPayload *)payload forType:(PKPushType)type withCompletionHandler:

If the app has a notification service extension, and the push payload contains mutable-content, and if the app is terminated, then the app gets launched in the background and the push still is delivered directly to the app, it does not get delivered to the extension.

If the push topic is changed to be just com.company.app (and provided the payload contains the mutable-content flag) then in that case it gets delivered to the extension. However now it's no longer a voip push, it's just a regular push, because the topic no longer contains the .voip suffix.

So how can things be configured whereby a voip push gets delivered to a notification service extension? Because that's what the Apple documentation says:

Call this method when your notification service extension receives an encrypted VoIP call request.

From: https://developer.apple.com/documentation/callkit/cxprovider/3727263-reportnewincomingvoippushpayload

How is that possible, how can a notification service extension receive an encrypted VoIP call request like it mentions in the documentation?