Secure Phone Authentication Solution Development

I'm looking for a secure alternative to Firebase for phone authentication on both Android and iOS. My primary concern with Firebase which is need to have blaze plan. I need alternative aside from firebase

Key Requirements:
- Development of a phone authentication solution that is secure and reliable
- Implementation of Two-factor authentication
- Cross-platform compatibility for Android and iOS

Ideal Skills and Experience:
- Expertise in mobile app development for Android and iOS
- Extensive experience with implementing secure authentication systems
- Knowledge of Two-factor authentication processes and technologies
- Ability to develop a solution independent of Firebase

And recode our program using the new phone authentication