Hello Amir,
Im not 100% on this, but I dont think phones themselves are MGCP, they either talk skinny or SIP. But anyhow, you have to TFTP the phone loads on to the flash of the UC500. From there, you need to have tftp-server statements to pull the files from flash, as the phones get their config via tftp. For example, this is off of our UC500 for 7940s and 7960s
tftp-server flash:P0030702T023.loads
tftp-server flash:P0030702T023.sbn
tftp-server flash:P0030702T023.sb2
tftp-server flash:P0030702T023.bin
Then, under telephony-service, you will put the phone load name, without the file extension. For example:
telephony-service
load 7960-7940 P0030702T023
From there you should be good to go.
Hope that helped, if so, please rate post.