it should have been included in the voicemail manual that gave you (and if you are like me, that went straight to the trash).
I'm sure it's on their website somewhere though. I know that for my area, it is the same first 3 digits, and then 6400. but yours could be different.
If you try this link, you'll be asked for your phone number, and then you'll be given the access code.
Regards,
ElizabethS