Edit the *97 dial plan and add a set action to "authorized=true" anywhere before the lua action.
Just be aware of the security risks. Anyone with access to call *97 from a given extension can access that voicemail box. Some places it won't matter, but something to keep in mind.
Just in case it is of help. Here is how I built my *97 dialplan to remove the PIN for certain extensions only. There are also some variables set to stop some other voicemail prompts: