VoIP QOS best practices

You are mistaking utilization for congestion. At the packet level, a link
is congested if it is not immediately available for transmit due to one or
more previous packets still being queued/transmitted. This transient
congestion causes jitter, VoIP's worst enemy.

Certainly, as utilization rises so will congestion; however, it is quite
common to have transient congestion while overall utilization is minimal.