I heard recently that Apple made it possible to developers to run their app in the background and grab information about the incoming call. Such as the phone number. This has been a security/privacy concern for many years, but I just heard that Apple officially made it possible to do so.
I searched for this question and I saw that it has been asked before some years ago, before the changes were made.
Possible use case would be to run the phone number against a private database and pull information about the caller before answering the call.