Replace the account ID with the extension number?

hello, Snom model D1xx , provisioned on Broadworks - the idle screen displays the Display Name (users name)
and beneath this is the account name,
i would like to replace this with the extension number, which i can do via 
"Display name for Idle Screen" on the web gui of handset (this updates the "User_Idle_Number1=" in the settings file).

will be deploying multiple handsets so dont want to log into each gui , What do i need to enter on the settings file so that this row will display the users extension in this field, pulled from broadworks?