Calculating the savings for moving from a conventional PBX phone system to a voice-over-Internet Protocol (VoIP) setup is anything but easy.
Why? Telephone usage varies, making it difficult to calculate savings. A company that makes many international phone calls may save more with a VoIP phone system than a company or department that mainly deals with customers and suppliers in its city or state. An office building's age and the robustness of existing data network hardware are other factors.
Another factor: the workforce's mobility. Do they work off-site with clients, cluster in project-based workgroups, or stay in one office for years?
A wide range of alternatives offered by vendors complicates the process. Options include full-blown Internet Protocol systems, hybrid systems that use the "old" PBX but add an IP-based software client to the desktop, and hosted services.
Not surprisingly, prices vary. Organizers of the VoiceCon 2005
conference issued a request for proposals for a hypothetical VoIP installation and obtained bids from five vendors: Aastra Intecom, Alcatel, Avaya, Cisco and 3Com.
The technologies offered by the vendors varied greatly, and so did their prices. The total cost of the systems bid ranged from an undiscounted low of $1.9 million (3Com) to a high of $3.3 million (Avaya). The moral of the story? When soliciting bids, be diligent and ask a lot of questions.
Thus, this worksheet should be taken as a rough guide. Make no mistake, your mileage will vary.
Tools: Calculators 
