I would like to know what are the port needed open by security or cross FWs for a remote Cisco IP phone (from remote cluster) can hear and access to Unity voice mail of the Called Phone.
Here is the doc that Gonz referenced. (+5 points for this buddy!)
This link shows the TCP and UDP Ports used on the various CCM Versions;
Hope this helps!
For audio you need RTP stream (UDP 16384-32767) and SCCP Ports signalling 2000 or if secure SCCP 2443
We have a CallManager and Unity ports guide available in CCO.