Whatch out about the bandwith. Most providers will give you a lot of bandwith from the net to the user (downoload direction), but just a little bit from the user to the net.
On VOIP ,you will need the same ammount of bandwith on both directions.
The normal cable service, like the one that I am using right now provides about 1000 kb, or 1 mb from net to user.
I only get about 350 kb the other direction.
The smallest number is what will dictate how many lines you could use simultaniusly.
128 kb should be O.K. for a Quintum analog A800, with eight lines.
Not top of the line hardware, but it works, have support and you could but it without breaking the bank account.
Good luck to you!!!!!!