He means the number you dial or speed call button you press to access your voicemail box/system (not the envelope key)... there is an extension assigned to the hunt group for the voicemail system, regardless if it is embedded or external, you need to dial that extension number (or press the programmed key on your phone) and after it answers enter *9999 followed by the password assigned in ESM to log into the mailbox.
If you don't know this extension, have a speed dial button for it, or know how to access ESM, I suggest you contact you authorized system administrator or Mitel vendor.