This is can be done in a variety of ways, but all would involve using a STAR Routing application and a few CRA's in the voicemail... if you are not familiar with this it can be a little confusing, in your Mitel 5000 go to Voicemail-Devices-Applications and press F1 to bring up the help and read up on STAR.
Even the most veteran technicians sometimes struggle with STAR applications, they are a top down, first match set of rules that point calls to other routing elements (such as Auto Attendants or Call Routing Applications) to handle routing of calls based on time, day, date, day/night mode or other specific requirements.
To be honest, I generally recommend NOT trying to do this by yourself the first time... there are many rules and tricks that can easily be gotchas that don't show up until it's too late since testing is difficult at best since it all based on specific date/time rules that are difficult to reproduce.