The traditional MAC savings generally are calculated on the basis of $100 for each calling change saved for larger businesses and $50 for each change for smaller businesses. These changes can be addition or deletion of new employees, alteration of access privileges (such as when a probationary employee becomes a permanent one) or other modifications such as a changed voicemail greeting, call forwarding, automatic call notification, the addition or revision of other contact phone numbers and e-mail addresses for employees.

Because most IP-PBXs allow configuration for multiparty (more than three parties) calling, third-party bridging services will be required less, if at all. Traditional bridging services can cost 40 cents or more per minute per line. A single, one-hour, 50-participant call can easily reach $1,000. Some service providers also insist on a 24-hour notification -- not practical for businesses with sudden, ad-hoc conferencing needs. With IP-PBX, the cost of that conference bridge is absorbed with just a few such sessions.

And, of course, there are savings on long-distance charges! A good VoIP plan should reduce your calling fees by at least half, and often more.

I often discuss these IP telephony ROI specifics in my What Works at Work features for Von (Voice Over Net) Magazine.