I would verify if the "Set Registration Access Code" in the System Options is really set to "***", the Device Type in the User and Service Configuration is set to "5224 dual mode", the Service level is not set to "Trusted" and the MAC Address is empty and that you are pressing Superkey (the blue button) and not the Hold key.
If this doesn't help I would try to do phone factory reset and also try to assign the phone some other COS with default values.
I checked the Registration Access Code multiple times its set to *** and the replacement to ###(the defaults).
Like mentioned the device type is set tot 5224 Dual Mode, Service level is Full and the mac address has been cleared several times.
This afternoon we will indeed do a Factory Reset to see if it will help, were expecting that there is a network issue.
We don't have their complete network drawing so were not sure if there is a extra switch or anything placed.
I just tested it with a Teleworker also a 5224 Dual Mode phone which works fine.
Only thing was I had to use the replacement code.