If Cisco IP Communicator is behind a firewall, make sure that the firewall is configured to pass TFTP and RTP traffic using the appropriate port range. See the "Selecting an Audio Port Range"

You might need to select an audio port range for Cisco IP Communicator to use if the network uses a custom port range for RTP audio. For example, if a single port is opened to allow audio to pass through a firewall or if a quality of service (QoS) policy has been applied to only those routers and switches with a restricted range of RTP ports.

You can do this from Cisco Unified CallManager Administration: Phone Configuration > Product Specific Configuration panel. Alternately, users can do this from the Cisco IP Communicator right-click menu: Preferences > Audio > Network > Audio Port Range.