The phone load that you currently are using on the UC520 should be upgraded to a newer version. In the example below I am using 7-4-8a.
First you will need to of course make sure the phones firmware file is in the flash.
Next add a tftp statement showing the location of the firmware such as:
tftp-server flash:/phones/50x-30x/spa50x-30x-7-4-8a.bin alias spa50x-30x-7-4-8a.bin
Next under telephony services change your load statement to complement the new alias:
load 504G spa50x-30x-7-4-8a
If you need further assistance with this please feel free to contact us at 1-866-606-1866.
Thank you,
Jason