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It can be, but it isn't as clear cut as it might appear. Keep in mind that while the data for VoIP calls is going over your existing nextwork, but you are going to have to invest in some equipment: an edge device for SIP traffic, a PBX and a SIP trunk from an Internet Telephony Service Provider (ITSP). After you invest in the equipment, remember that you are sending all of that voice over your internet channel, so depending on the size of your business you may or may not have to address that. The load isn't really all that much, about 80kbps per phone channel. What is really important is the quality of the calls, and that requires prioritization of voice packets vs. data. One way is to set up a VN dedicated to voice traffic, which can add expense, as well as locking away bandwidth that cannot be used for data, even when there are no calls in progress. Just make sure that you have enough bandwidth to do it. After you get all set, I would expect the monthly cost per line to be no more than half of what it would be from a traditional PSTN (Public Switched Telephony Network).
One huge benefit if your company relocates is that a VoIP system allows easier portability of your phone system. A friend of mine recently moved her business across town, and the biggest headache she experienced was getting her land lines up and running. That business had no functioning telephones for almost two weeks after the move. You would think that after 100 or so years the phone company would pretty much have it down, but apparently not, or at least not always