iOS 13 will bring a few new features to iPhones upon its release later this year, several of which are focused on protecting user privacy. One such update will block background processes from collecting user data, something that several messaging apps like WhatsApp Facebook Messenger apparently rely on for receiving VoIP calls.
https://www.notebookcheck.net/Apps-like-Facebook-Messenger-and-WhatsApp-may-need-redesigns-thanks-to-iOS-13-s-background-access-restrictions.428809.0.html