You can use an external TFTP server to off load the TFTP server function on the Cisco Unified CME router.
This is also important if your router is not a high end router while the CME configuration is complex with many phones and expansion modules. Also Using flash memory or slot memory on the Cisco Unified CME router allows you to use different configuration files for each phone type or for each phone, which allows you to specify different user locales and network locales for different phones
To define a location other than system:/its for storing configuration files for per-phone and per-phone type configuration files, perform the following steps.
telephony-service cnf-file location flash: cnf-file perphone
The above configuration did resolve an issue with one of my customer where all the phones would reboot after 5 minutes.
Note: When the storage location chosen is flash memory and the file system type on this device is Class B (LEFS), check free space on the device periodically and use the squeeze command to free the space used up by deleted files. Unless you use the squeeze command, the space used by the moved or deleted configuration files cannot be used by other files. Rewriting flash memory space during the squeeze operation may take several minutes. Run this command during scheduled maintenance periods or off-peak hours.
Reference: Cisco CUCME Admin Guide