for a single channel, you are correct IF you make a "deal" with a land line provider (which is the same deal you could cut with a VOIP Provider). There are "unlimited minute" plans available all over the place.
But: All "unlimited minute" plans include a provision that outlaws dialers. It may be hidden in a clause similar to a "fair use" section. It may be denied entirely by the salesman. You may never actually "see" this agreement. But rest assured, if you use a dialer to dial more calls than a human could dial, you are violating the agreement with your land line provider and/or your VOIP provider unless you are paying per minute. Many of these providers have a clause that will allow them to "back-bill" you for the minutes you used in violation of the policy. and this is fair, after all you are using automated equipment to circumvent an otherwise fair agreement. Try using automated equipment to beat the Casino and see how funny they think it is. They are there to make money on your calls, not lose money.
this applies to Magic Jack, Skype, Vonage, MCI, AT&T and all others that offer "unlimited".
Plus: a single line or single channel of unlimited minutes is not likely to even keep ONE agent busy unless you are dialing business to business and always get an answer. So you'll need two (or three?) unlimited lines per agent.
Then, of course, is the return calls. When you get a call back on an "unlimited minute" line, it will not have the ability to "roll over" to another line unless specifically set up that way (business class multiline with hunt ... hunt costs extra). So your return calls may experience busy signals constantly unless you block your number. It may be possible to change your callerid to another number (with multiple channels for inbound ... or perhaps just a line reserved for inbound that is kept available by keeping all inbound calls short ... but that's not a good way to do business).
It is possible to sustain for one or two agents in a short-term situation when money is very tight and time is abundant. But for the long-term, it is not sustainable. IMHO
Happy Hunting, though.