I'm building a mobile app and looking for a reliable, cost-effective way to verify a user's phone number when they add or update it in their profile. This is not for login or authentication; login is already handled through Google Sign-In and Apple Sign-In.

On Android, we're using Google's Phone Number Hint API, which allows users to select a number associated with their SIM, although it doesn't actually verify ownership. On iOS, Apple provides Phone Number AutoFill, but it only suggests saved numbers and doesn't offer an equivalent SIM-based selection or verification API.

We've considered Firebase Phone Authentication, but since OTP requests are initiated through the client SDK, enforcing strict server-side rate limits and preventing SMS abuse is a concern. Twilio is another option, but costs can add up, and we're also trying to avoid the DLT registration requirements and associated costs in India.

Has anyone faced a similar situation? Are there any reliable, affordable alternatives for verifying phone number ownership, particularly on iOS? We'd love to hear about real-world implementations that work across both Android and iOS, especially for users in India and the UAE.