If the user continues to refuse accepting, they will eventually lose access to calls, and will no longer receive notifications, rendering the app practically useless.
Quoting the App Store guideline 3.2.2 (vi):
Apps should allow a user to get what they’ve paid for without performing additional tasks, such as posting on social media, uploading contacts, checking in to the app a certain number of times, etc. Apps should not require users to rate the app, review the app, watch videos, download other apps, tap on advertisements, enable tracking, or take other similar actions in order to access functionality, content, use the app, or receive monetary or other compensation, including but not limited to gift cards and codes.
Therefore, WhatsApp’s actions are not allowed under the App Store guidelines, which could result in the removal of the app from the App Store, or termination of their developer account, according to Apple’s policies.