Auto-Attendant recordings must be performed at the Attendant Station. This is typically extension 100, but your system may have been programmed differently. Press  from any phone to ring the Attendant Station.
Auto-Attendant Greeting Recording
1) Press the [Trans/PGM] button, then enter  + 
2) Enter the announcement number (001-100) that you wish to record.
3) When prompted to “PRESS REC TYPE (1-6)” press 
7) Press the [Speaker] button to quit.