Our app uses Firebase phone auth. The App Store review has rejected the app due to pushes being required:
Guideline 4.5.4 - Design - Apple Sites and Services
We noticed that your app requires push notifications in order to function.
Specifically, we noticed if the Push Notifications setting was not enabled on our device, we encountered an error message after entering our phone number within the app.
Push notifications must be optional and must obtain the user's consent to be used within the app.
To use phone number authentication, your app must be able to receive APNs notifications from Firebase. When you sign in a user with their phone number for the first time on a device, Firebase Authentication sends a silent push notification to the device to verify that the phone number sign-in request comes from your app. (For this reason, phone number sign-in cannot be used on a simulator.)
4.5.4 Push Notifications must not be required for the app to function, and should not be used for advertising, promotions, or direct marketing purposes or to send sensitive personal or confidential information.
Firebase support has confirmed this bug and has given the response to this issue:
Thank you for the detailed information Leet. This is indeed an odd situation, and our Phone Auth engineers are taking a look. I'll get back to you as soon as I have an answer.
Our engineers are working internally on a long-term fix for this, but in the meantime, would like to meet with you to see if we can figure out a workaround.
It appears that there is indeed a bug in Firebase phone auth / Firebase UI.
While the only solution currently is to wait for Firebase team to fix it, there is an alternative service - Fabric Digits (which Firebase phone auth is aiming to replace). After purchasing Fabric, Google has announced that Digits will be discontinued on September 30th, but untill that time it will remain functional.
We have initially developed the app using Digits, then migrated to Firebase phone auth only to be rejected, and thus reverted back to Digits. Lets hope Firebase team fixes phone auth before September 30th.