NoName is correct if we are talking about inter-office calling is the consideration. To determine moving to VoIP vs. Upgrading existing Telephony equipment one must consider all avenues. Do you have a 800 line for calls routed to your office?
(Cost Savings moving to VoIP here)
What is the primary business your company does? What is the cost to upgrade vs. moving to VoIP?
I feel that making the choice when moving to VoIP could have various cost savings depending on who you chop it up.
Let me know if you would like to have a phone conversation regarding this matter.