I am looking into VoIP phone plans right now. I find that there are "unlimited business plans," but I wonder if there are realistic limits on this. There are places like www.broadvoice.com that have these plans, but I am wary that the phones will get cut off for excessive usage (I expect each line to be in almost constant use).

Right now I'm using a place that charges 2.9 cents per minute, I can knock that down to 2.5 or so cents per minute if I buy a $1,000 block of minutes... with higher discounts as I buy larger blocks.

Is there a better way to do this? Anyone have experience in this area? All calls are made to and from the US.
Also, if anyone has experience with regular land-lines, that'd be helpful to.