Resolution
TCP ports 1099 through 1129 are used for the Remote Method Invocation (RMI) connection between the Cisco CallManager Attendant Console (AC) client and the AC server (the first available port is used). Port number 2748 is used to send Quick Buffer Encoding (QBE) messages to the Computer Telephony Integration (CTI) server.
User Datagram Protocol (UDP) ports are mostly used for line state. The UDP port can be configured with the Advanced Settings dialog box. If no port is configured, the AC uses the first available UDP port randomly.
If the AC uses TCP ports which fall outside the mentioned range, refer the document In Cisco CallManager 4.x, the AC fails to work across a firewall, with the error message "error communicating with the server ports" and uses random TCP
For more information, refer to these documents:
