your lines are assigned to Line Groups and by dialing 9 it is trying to pick up the first available line in that group. If you want to go down to just that one line you can take the other line out of the trunk group. Your Loop keys are just Pooling from that Trunk Group.

You could really program the lines on to keys so in the event that your are experiencing you can just touch the other line that is working to use for outbound access

Hope that helps!

Gary