You can develop and test applications using WhatsApp, Viber, or Facebook Messenger through the Messages API Sandbox, which allows you to link your own accounts and start sending and receiving test messages on your devices as you build. To get started, you need a Vonage API account, a smartphone with one of these messaging apps installed, a command line or terminal application, and whitelist yourself by linking your account ID to the API Key of your Nexmo account. Once whitelisted, you can test your account by sending yourself a message using the Terminal application, and then connect your application via webhooks to receive inbound messages from any of the supported services. The Messages API Sandbox provides tools such as Status webhooks to monitor the delivery path of messages sent by your application, allowing you to build applications that interact with these messaging platforms quickly and easily.