What code is used to program a voicemail button on a Vertical SBX IP system (8 Button telephone)?

I looked in the User Guide on page 2-65 and I found this:

1. Press TRANS/PGM, then press the flex button you want to program.
2. Press TRANS/PGM again.
3. Enter 47, then enter the number of the mailbox to be associated with the selected Flex button.
4. Press HOLD/SAVE to save the setting.

See if it meets your needs.

User Guide