I am looking for Passwordless PhoneNumber-OTP based authentication for my flutter mobile app with Spring Boot as backend which will run on AWS EC2. I used Firebase Auth before which is very easy to use and does all these OutOfBox. I understood Cognito doesn't support this but has custom challenges to do. But seems, to send OTP logic in custom challenges lambda needs to pay for some other service like Twilio SMS where Firebase sends these OTPS to Phonenumber for free!
Is there any ideal way to achieve this? There are quite a few docs/articles on this.
Would be great for any insights.