The ports are in use when someone hits the auto attendant, when someone`s leaving a message, or when someone`s checking a message. (or doing voice mail programming) You could have many users in the voice mail, as they wouldn`t all be checking messages at the same time. If you only have 2 sets installed, you`ll have to get 2 outside callers to hit your AA, and have your 2 sets access their mailbox, all at the same time. Four sets and you can do what DueE said. Remember too what Phone Man said about the daughterboard. A 2 port flash could have 2 stations wired, (why someone would wire the second one if its not a 4 port is beyond me, but you never know) but the second one will only work if you have that daughterboard installed. You`ll see the number 4 on the daughterboard when you open the cover if its a 4 port. A 2 on it and its just a blank cover.
Schlum