ATA 186 Refusing to Register to New Server

Whilst working on a CUCM merger project, the process below worked for moving regular phones from the 2 existing CUCM clusters over to the new CUCM cluster:

1. Set DHCP scope option 150 to new CUCM server's IP address
2. Bulk erase ITL files on all phones using a 3rd party tool
3. Reset all phones

However all the ATA 186s flat out refused to register to the new CUCM despite multiple attempts. Looking at the ATA's built in web server showed the correct TFTP server had been picked up from DHCP. Eventually found that you can force the CUCM IP addresses to register to in the GUI:

Reset the ATA 186 after applying the change & voila it works!