Cisco SPA504G picks up the device ID in the $USER field when Asterisk is in Device & User mode. The LCD on the phone shows the device ID and not who is logged in. Is there a different Asterisk (Cisco?) field name that has the ID of the user who is logged into the device?
Cisco SPA504G Firmware 7.5.5
You can not determine who is logged into a phone from the phone. It’s not possible.
How about putting scripts on a PC/server that users would run to cause the phone to resync to a config file that logged in as the user and displayed name and extension #? This wouldn’t need to be in device and user mode and users would “log in” from the PC instead of the phone.