Determine DNS or Connectivity Issues
If the phone continues to reset, follow these steps to eliminate DNS or other connectivity errors:
Use Erase to reset phone settings to their default values. See Cisco Unified IP Phone Reset or Restore for details.
Modify DHCP and IP settings:
a) Disable DHCP. See Network Configuration Menu for instructions.
b) Assign static IP values to the phone. See Network Configuration Menu for instructions. Use the same default router setting used for other functioning Cisco Unified IP Phones.
c) Assign a TFTP server. See Network Configuration Menu for instructions. Use the same TFTP server used for other functioning Cisco Unified IP Phones.
On the Cisco Unified Communications Manager server, verify that the local host files have the correct Cisco Unified Communications Manager server name mapped to the correct IP address.
From Cisco Unified Communications Manager, choose System > Server and verify that the server is referred to by its IP address and not by its DNS name.
From Cisco Unified Communications Manager, choose Device > Phone > Find and verify that you have assigned the correct MAC address to this Cisco Unified IP Phone. For information about determining a MAC address, see Cisco Unified IP Phone MAC Address Determination.
Power cycle the phone.