Is there a sandbox environment for a customer to test the Sinch WhatsApp Business APIs before going live?

Yes. A simple tutorial and associated sandbox to enable a developer to send their first WhatsApp business API message are available: https://www.sinch.com/blog/whatsapp-business-api-tutorial/

The Sandbox uses a WhatsApp Account and MSISDN owned by Sinch so this testing can happen before the WhatsApp validation of a customer's individual account.

Learn more about how you can send WhatsApp messages to your customers easily and securely using the Sinch WhatsApp Business API.