Port' refers to the number of simultaneous connections between the voice mail system and the phone system. The caller will generally hear ringing or busy tone - depending upon the configuration of the phone system.
Some phone systems allow overflow calls to ring at a receptionist phone.
Determining the number of ports required for a system is an art, not a science. Some people, like real estate agents and lawyers, are heavy voice mail users and always require more ports. As a general rule, 2-port systems should be avoided except in homes or very small offices. Small offices with voice mail only during the day and automated attendant only at night may get by with four ports. Any office using more than a few telephone lines and expecting to use automated attendant during the day should install more than four ports.