R1 - is it a module in one of the slots of the cabinet? If so, it is "VS" mail. Log into mailbox 99, press 9 for programming, press 3 for auto attendant, press 1 for day or 2 for night, then 6 to modify. IF you are changing the DAY message, press *#, for the night message just go to the next step. Press 1 to record a new message, wait for the tone, speak, press 1 at the end, then 23 to play back, 21 to rerecord, and FINALLY press *# to approve the message.
Now if it is a PC type bpx on the wall next to the phone system, post back, cause that one is a little different...