I would say first, find an authorized phone number verification provider. Don't trust any random service on the internet.
They probably would have an API for Android and iOS or a rest API.
If a rest API is available, you can create a client in commonMain source code using ktor client and use this client in common, it will serve all platforms.
Otherwise, if only Android and iOS sdks are provided, then you would create a PhoneVerifier interface/abstract class in commonMain and provide an implementation in every platform wrapping the afore mentioned sdks respectively