Are you deploying VoIP in a traditional PBX environment? With a state-of-the-art IP PBX? In a campus environment? Across dozens of locations worldwide? Need to support fax and other analog equipment? Want to "hop off" and/or "hop on" your network for long distance bypass?
Most importantly, do you want to ensure that the reliability of your voice services won't be jeopardized by problems on the network or the failure of any single piece of hardware?