The scan has found no failures. Am I safe?

The scan has found no failures. Am I safe?

Not really. The scan attempts to find as many security vulnerabilities in your SBC as possible, but it does not claim to find 100% of issues. Also do note that attacks can happen at any time, even immediately after the scan completes.
Security is a discipline rather than a one-off activity. The threat landscape is always changing – new attackers emerge, new methods are employed, networks change with new connections, calling patterns change. In short, the state of SBC security is always changing. You need to scan periodically to have a robust and secure SBC.