for HW , you can direct by FTP or SCP

it may  use the command  "show call active voice" and "show call active voice brief" to check with IP address between the call leg, it may firewall block RTP or routing problems with End to End RTP. Hope its helps.

I hope this can help, https://www.cisco.com/c/en/us/support/docs/security/cloud-email-security/215858-downloading-logs-from-the-gui-of-your-es.html

If Control Center showing wiht Add-on , that mean CCX engine are not start on that server.