I think the multi-line feature is referring more to the fact you can hook up a Voice board and connect several physical analog lines to IVM. It may also be referring to VoIP lines (in VoIP each 'line' can have several channels or 'virtual lines' linked to it).
I'm not exactly sure how one physical line can ever allow more than 2 simultaneous call though. To be more technical, almost every standard phone wire has the ability to carry up to two phone lines. In a normal set-up, the extra 2 smaller wires inside the phone wire aren't used. In a 2-line set-up, 2 of the wires are used for line 1 - the other two for line 2. This is how a multi-line phone usually works (makes use of all 4 wires inside the phone wire and requires 1 phone wire per every 2 lines). I'm not aware of it being possible to have more than 2 calls on one physical line without some special cable/equipment.
Instead, I'm pretty sure the line would have to overflow to another physical line if the first one is busy. I'm still learning about how analog phone systems work (my focus is more on digital), but I'm pretty sure this is how most set-ups would be. One phone number would be linked to line 1 which would automatically forward to line 2 if busy, which would go to line 3 if busy and so on.
Your telephone company does not face this restriction. Because of the way their equipment operates, your phone company can easily forward all calls to Voice Mail for your line without problems, even if there are several simultaneous calls. If you have an answering system on your end however, you are faced with these line restrictions, and if your lines are busy, your phone company would automatically forward calls to your old Voicemail, or play a busy tone if you un-subscribed.