A sandbox is a non-production environment for building and testing an RCS integration, where messages reach only approved test devices and nothing goes to real customers.
What an RCS sandbox contains
In RCS Business Messaging, an agent exists in a test state before it is launched. It has an agent ID, a brand profile and the full API, but it can only message phone numbers added to its allowlist of test devices. That is the sandbox: developers send rich cards, carousels and suggested actions to their own phones, watch the webhook events come back, and exercise the SMS fallback path, all without a launched agent or a registered 10DLC campaign.
What moves an agent out of it
Launch. Google reviews the agent's brand, use case and sample messages, and the carriers approve it for their subscribers; after that the allowlist no longer applies and the agent can message any opted-in customer. The verification process is where most of the calendar time goes, which is why it should start before the integration is finished rather than after.
Sandbox versus a live test
A sandbox tests your code. A live test from a launched, verified sender shows what your customers will actually see, name, logo and checkmark included. SimplyRCS offers the second kind free: send yourself a message before you build anything.