Option A would be the best method to validate the possible findings. Manually checking the version number of the VoIP service against the CVE release would provide a direct and reliable method to confirm whether the CVEs apply to the system.
Option B might provide some additional confirmation, but it is also risky as the proof-of-concept code could potentially harm the non-production system, and might not necessarily provide a conclusive result.
Option C might be useful for detecting ongoing attacks, but it wouldn't necessarily provide validation for the presence of the identified CVEs.
Option D, an nmap -sV scan, might help to determine the version number of the service, but it might not necessarily provide definitive proof of the presence of the CVEs. Therefore, option A would be the best method to validate the possible findings.