Sinch SDK which is available for iOS and Android, allows you to add voice calling and instant messaging to your app in minutes. The SDK features the ability to call app to app, app to phone calling, instant messaging and SMS to your app. The service is free for the first 25,000 users and you pay as your business grows.
Sinch makes it simple by hosting everything in the cloud and is available 24/7. With Sinch you can use our existing UI for your app to offer customers a seamless experience. Check out Sinch HERE