check what is the TFTP server address that the IP Phones are trying to use. On one of the phones, go to settings, then network
configuration, and look for the TFTP IP address. Does it match the correct IP address of the TFTP server?
- Please make sure that there is good IP connectivity between the IP Phone and the TFTP server. You can go to the TFTP server and as a test, do a ping to one of the phones that are failing to make sure it responds.
- If it responds to the ping, is it possible there has been any changes that may prevent TFTP traffic to pass from the server to the phones?
Check VLAN, dhcp and also the ip source address under the telephony service
