Like other IP devices, the Cisco ATA receives its configuration file and list of Cisco Unified Communications Managers from the TFTP server. If the TFTP server does not have a configuration file, the Cisco ATA uses the TFTP server name or IP address and port number as the primary Cisco Unified Communications Manager name or IP address and port number.
After the Cisco ATA initializes, both ports on the Cisco ATA (skinny clients) attempt to connect with the primary Cisco Unified Communications Manager. If the connection or registration fails, the Cisco ATA skinny clients attempt to register with the next Cisco Unified Communications Manager in the Cisco Unified Communications Manager list. If that connection fails, the Cisco ATA skinny clients attempt to register with the last Cisco Unified Communications Manager in the list. If all attempts to connect and register with a Cisco Unified Communications Manager fail, the client attempts to connect at a later time.
Upon successful registration, the Cisco ATA client requests the Cisco Unified Communications Manager software version, current time and date, line status, and call forward status from the Cisco Unified Communications Manager. If the Cisco ATA loses connection to the active Cisco Unified Communications Manager, it attempts to connect to a backup Cisco Unified Communications Manager in the Cisco Unified Communications Manager list. When the primary Cisco Unified Communications Manager comes back online, the Cisco ATA attempts to reconnect to it.