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.
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Good luck to you!!!!!!