User A is attempting to call User B. Both phones are registered to Cisco UCM. User A tries to add a third caller to the conversation but cannot complete the conference. Calls from User A into a…

User A is attempting to call User B. Both phones are registered to Cisco UCM. User A tries to add a third caller to the conversation but cannot complete the conference. Calls from User A into a Cisco Meeting Server space are working. Upon troubleshooting, it is discovered that the issue is caused by Cisco UCM not trusting a certificate. What certificate must be trusted by Cisco UCM to resolve this issue?