Some of the older UC520 systems were shipped with other firmware versions for the 521 and 524 phones. It seems there are a number of feature upgrades with the newer firmwares. One of which is the ability to enable the switch port on a 524SG model phone.
You should load at least 8.1.13 or higher firmware on the system.
Don’t forget to setup the tftp-server and load commands so that your phone updates.