There are many VoIP systems and options available and needless to say, what you and your organisation should purchase ultimately depends on your requirements and plans for growth.
Initially it may make sense to introduce VoIP as an addition to your existing phone system (assuming you have one) and then gradually increase your level of sophistication as and when you need more functionality.
For many small businesses, perhaps a managed or hosted option might be the preferred option, but for larger organisation, it is unlikely that this would be adequate.
The key issue here is whether VoIP is best for your company. Make sure you fully investigate traditional call services before taking the plunge.