I have used CBC effectively for this, setup one of your charts to show agents available for the voicemail group - if it ever gets down to 1 agent (or worse 0) at any point in the day, you should think about adding ports.
In case you weren't aware, you can (and should) drag your mouse pointer across a section of the chart to zoom in... otherwise you can easilly miss very brief periods when the port availability drops way down.
Peter