Regarding only Zoiper for iOS released after 3.25.2, previous versions are not affected.
Due to an update from Apple regarding handling of Push notifications in iOS 13, starting April 1st 2020, all apps must handle push notifications as per iOS 13 development framework.
Therefore, you might be experiencing some issue when it comes to Push notifications in Zoiper for iOS and NOT receiving Incoming Calls.
If you get a WARNING: PUSH NOTIFICATIONS error, or you want to preemptively be prepared:
Go to iOS Settings > Notifications > Zoiper and turn on Allow Notifications option.