If you reach the auto attendant when you call in, press "*7". You will hear "Welcome to Avaya Merlin Messaging" or something similar.
Next you enter your extension number and "#", and finally the password followed by the "#".
After you are finished you can either hang-up or [my personal favorite] press "**9" to disconnect the call.
If a live person answers the call have them transfer you into your voicemail.