Infoworld recently posted a great story detailing this exact problem and just how bone-headed IP telephony implementation can go, and how a simple mistake (like mis-calibration) can make an entire technology seem deficient.
In the story a VoIP service provider was called in to an organization to check out their communication system. The organization had been experiencing a lot of crippling problems with their IP PBX. The organization’s employees were experiencing degradation in call quality, dropped calls and busy signals when and where there shouldn’t. In other words this organization’s VoIP deployment has turned into a mess in a manner their old traditional telephony solution never had been.
Turns out they were using an IP PBX that wasn’t nearly powerful enough to meet their current needs.