I don't believe in any formula when it comes to voicemail ports. I have used 2 port voicemail systems for small offices such as a doctors office with live answer. The only time the ports are in use would be when someone is leaving or retrieving a message only, and where the auto attendant in not in use. Whether or not you are using auto attendant is key to the number of ports. If people pick up transferred calls then the ports are in use a short period of time. If messages are being left and then retrieved the ports have much more usage. If you use voicemail the way I do usage is even greater. If you call me chances are you will have to leave a message, then my voicemail will call my cell phone and I will listen to the message. If you have a lot of people doing this usage becomes much higher. In call centers I use even more ports since you are holding people in queue waiting for an agent. So in my opinion it depends on how it will be used first and then size of the stations/trunks second.
