It's hard to reproduce a missed or delayed call. So I used some scripts to periodically sent some messages targeted to certain devices. I just realized that there were some mistakes in some device tokens which led to some missed calls. So those statics were not accurate.
But I do want to know how reliable the VoIP push is. Is it 100% guranteed to be delivered at a timely base? If not, how long could it be delayed? Or what's the percentage of failure to deliver.
Push notifications, VoIP or otherwise, cannot be guaranteed to be received by the target device 100% of the time without delay.
Although APNs in itself is quite reliable, the last mile, the connection from APNs to the device is highly dependent on the quality of the device's connection.
In the cases where it was not delivered, or delayed, the device could be off, in airplane mode, out of reach (in a tunnel, on a mountaintop, etc.), or in one way or another have a poor quality connection.
Additionally, with statistics like yours, you would also need to consider where the break is. Assuming you have these stats based on the call home data from your app, it is also possible that although the push is received, there could be a problem (timeout, your server not accessible from the network node of the device, etc.) reporting back. Therefore the stats you have are likely for missing callbacks, not missing pushes. To start suspecting an APNs wide issue for your app, you should make sure where the problem is to start with.
In almost all the cases we have seen, the problem has been with either the app's push server (for example, mismatched tokens), the target device did not have a good network connection, or there was just some basic misunderstanding of how APNs works.
If you do suspect a widespread issue for your pushes, our server team can have a look, but being able to have specific data on missed/delayed pushes would be necessary.