SMS has become a ubiquitous method for sending notifications, often replacing more personal forms of communication like phone calls or emails. However, with the rise of asynchronous programming and the need to make urgent calls, there's a growing interest in using voice APIs to broadcast messages to users. The Nexmo Voice API is one such solution that allows developers to create custom voice broadcasts using various programming languages. By leveraging async/await syntax and libraries like aiohttp and motor, developers can build scalable and efficient voice broadcasting systems. In this tutorial, we'll explore how to use the Nexmo Voice API to make synchronous and asynchronous voice calls, with a focus on implementing backoff mechanisms to handle rate limiting issues.