1) A gateway to do the conversion betwen normal telephony to VOIP.
Will connect betwen your data world and the phone system (PSTN or internal PBX system)
2)Enough data bandwith to carry those conversations. Use internet broadband if you do not have a internal data network.
3)Good support from your PBX/phone people to do the interfacing of your existing phones to the new VOIP system.
The key questions: How many lines will you need????
How many locations involved???
Do those locations have already a phone system???? What type, PBX or KEY system,???? What kind of interface do they need???? T1, E1 ,or plain analog 2 wires lines???
Do you have internet or internal data network at those locations??? How much bandwith on both directions???
Could you ping one location from another in les than 600-800 ms always????
Those will be the main questions. One you have all htose details, almost anyone that works on VOIP should be able to help you to design a system.
And the most important: How much are you able to expend????