Avaya Voicemail Pro
Avaya Voicemail Pro licences are also called preferred edition, voicemail pro had been part of the product set since it was the IP400 systems and you will often find a small office system with Voicemail Pro licences.
Voicemail pro comes with 4 ports but you can add additional up to 6 2 at a time, the Voicemail GUI is comprehensive and you can create complicated call flows and database integration that you cannot do with the embedded voicemail.