Confirm your Cisco / Polycom Endpoint shows successfully registered to specific Lifesize GK IP.
Confirm your Firewall DO NOT have H323 inspection enabled. Alway best recommended to disable this feature to avoid call drops / jitter and packet loss.
(For e.g: if this is a cisco ASA firewall, please refer H323 Inspection)
Ensure the recommended ports are allowed from your Network : Third-party device registration
If still unable to place calls, attempt a call from your registered endpoint and open a Lifesize Support Request
If this is your Cisco Endpoint, from the Cisco Device WebUI,
Maintenance > System Logs > click Start Extended Logging > select Include a full packet capture
Attempt to place a call. followed by the call failure, from the Maintenance > System Logs > click to Stop Extended Logging, followed by click extendedlogging.pacp to download the file and attach to the Lifesize Support Request.