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For VoIP, what your normally want to do is insure the bearer traffic (the actual voice portion) gets first priority to your interface's resources (bandwidth and buffers). Voice signaling should be insured it's not excessively delayed and drops are avoided. For QoS to guarantee this type of service for these two types of VoIP traffic, you need to insure there's sufficient bandwidth. (Remember QoS allows interface resource management, it doesn't create bandwidth.)
PS:
Paolo says you don't need QoS on LANs. Well often that's true, as LANs generally have more bandwidth than WANs, but LAN interfaces can congest too. When they do, either you'll need more bandwidth (which often can be a viable solution for LAN) or you need QoS to manage your interface resources. I.e. sometimes QoS is very critical to meeting application service level requirements.
