Ok guys; This is for all you telephony buffs. ?

I was wondering the other day about VOIP and interfacing between Analog phone system (like a 4 pots nortel). Now I've used ATA adapters before but their only good for one or two lines... What if you wanted to convert an entire analog system to VOI P? I've look everywhere for a multiple port ATA adapter (maybe 4 port or more) but can't seem to find anything... Maybe I'm going about the logic the wrong way... Any help would be greatly appreciated... Thanks!