The Nexmo Voice API uses Nexmo Applications to hold security and config information needed to connect to Nexmo endpoints. A virtual phone number is required to purchase one that meets your needs. The Nexmo Voice API can be used to build a single level IVR with multiple paths, such as forwarding the call to a specific destination or playing a recorded piece of information. To create an IVR, you need to expose your local server to the internet using a tunneling service like ngrok and install the necessary nodes in Node-RED, including Nexmo and Ngrok nodes. The IVR can be configured to send caller numbers to voicemail or SMS with online booking links. The Nexmo Voice API provides a range of actions available, such as connecting callers, sending messages, and recording calls.