Late and missing Voip Push notifications

Our application relies on VoIP Type push notifications for incoming Calls. Calling is the major functionality of the application.

The VoIP push allows the mobile phone to:

• Start, launch the application if it is killed or in the background 

• Start the application if it is in the background or the phone is in deep sleep.

We conducted a series of testing and observed that there is a delay in the delivery of VoIP push notification to the mobile device.
This delay is about 20+ seconds and sometimes never arrives on the device.
We observed the delay in both cases when the iPhone is on Wifi only as well as when it has a sim-card.
During testing, we ensured the push was actually successfully dispatched to APNS.

Could you provide some explanation on why the VoIP pushes are delayed despite they are supposed to be instant? And how can we solve this issue?

Best regards,