This depends on a number of factors. To start with
1. How many users will you have?
2. How many locations do you plan on involving?
3. Will you require telecommuters to have access?
4. Do you have existing Telco contracts and when do they expire?
5. Do you have in-house expertise to manage a Voip Environment?
6. What is the current state of your network i.e. is it Voip friendly or will upgrades to the network be required?