The Vonage Network Registry Sandbox is a secure environment that allows developers to explore the Network APIs from the beginning, ensuring user safety and CSP protection. The Sandbox uses live data and enables up to 5 test numbers with allowlisted phone numbers, while also requiring authentication codes via SMS for verification. To use the Sandbox, developers must first allowlist their desired phone numbers and then create a new application linked with the Sandbox, generating public and private key pairs for JWT creation during authentication. Once configured, developers can begin testing Vonage Network APIs in this secure environment.