Hi Randall,
A couple of things to look at here. Is there a message on Line 2 when this happens? If so you may have this setting (set to false)
"Always Use Prime Line for Voice Message". This feature will select Line 2 or others if there is an actual Voicemail Message waiting (with flashing VM Icon) on the secondary lines.
Services > Clusterwide Parameters (Device - Phone)
Always Use Prime Line for Voice Message - True/False
Maybe a routing rule in Unity?
Hope this helps!
Rob