It is not necessarily so, but I can think of several reasons that is *can* be.
One way that it is cheaper (at least in the US) is that the service providers don't have to charge all the taxes they do for a traditional circuit.
For example, with a PRI circuit vs. SIP trunks both delivered via the same T1 transport with the same features and rates the SIP trunks can be $50 - $75 per month less because various taxes and fees can be avoided.
It is not likely to stay this way for long, but it is this way for now at least.
Toll bypass or hop-off routing is another advantage of voip and can lead to obvious savings if you can avoid long distance charges, but it is not unique to voip as you can do it with leased lines as well (as has been done with traditional PBX for decades).
Enabling remote users can be a cost benefit because you can place a phone somewhere on an existing internet connection where previously you would have to pay for a phone line or leased circuit to provide a phone.
Another way is that when you purchase a new phone system you can save the cost of T1/E1 cards. Depending on the system some may or may not charge a license fee for voip trunks.
Yet another way you can consider voip to be saving money is on network and cable infrastructure. You can choose to use single cable drops for phones with PC plugged into the back of the PC. If you are pulling new cable or would otherwise need to add cable this saves money. If you run voice and data over the same WAN link you can consider that you are saving the cost of purchasing separate circuits, routers CSU/DSU, etc.
If the voip system happens to be easier to administer then an existing legacy PBX then this may be a cost savings where the IT group can do moves and changes where previously you had to pay a phone vendor to these things.
Those are most of the common reasons I have heard people argue that "voip is cheaper"
Brandon
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