If you go to the system help and search on Non-verified it tells you that the input method for the 5212 (and 5215 for that matter) is different to that of a 5220 and 5224. It's around the higher end phones having better direct access to features. See below
Are you trying in input it before of after the call is in progress?
Operation
2500, SUPERSET 401, SUPERSET 401+, SUPERSET 410, SUPERSET 420, SUPERSET 4001, SUPERSET 4015, SUPERSET 4025, and SUPERSET 4125 telephones; 5001 IP Phone, 5005 IP Phone, 5010 IP Phone, 5020 IP Phone, 5201 IP Phone, 5205 IP Phone, 5207 IP Phone, 5212 IP Phone, 5215 IP Phone, 5215 IP Phone (Dual Mode), 5302 SIP Phone, and TeleMatrix 3000IP Phone:
To use Forced Account Codes:
Lift the handset.
Dial the Account Code digits.
If required, do one of the following:
To submit the Account Code, press #.
To cancel, press *.
To enter an Account Code during a call:
Press Trans/Conf () or Flash or flash the switchhook.
Dial the Account Code Feature Access code.
Dial the Account Code digits.
If required, do one of the following:
To submit the Account Code, press #.
To cancel, press *.
Press Trans/Conf or Flash or flash the switchhook.
5220 IP Phone and 5220 IP Phone (Dual Mode), 5224 IP Phone, and Navigator:
To use Forced Account Codes:
Lift the handset.
Dial the Account Code digits.
If required, do one of the following:
To submit the Account Code, press #.
To cancel, press *.
To enter an Account Code during a call:
Press SuperKey.
Press the Yes softkey.
Dial the account code digits.
Press the Save softkey.
Do one of the following:
For a verified account code, press the Yes softkey.
For a non-verified account code, press the No softkey.
