After fifty5's last post I went back and looked at this thread. I may have missed it, but I can't find anywhere where it says the outside calls are being answered by an auto-attendant, but it seems that would seem to be the logical assumption here. However I wonder it that is what is happening. Are the outside calls being answered by an auto-attendant?

It seems to me the way to make this work with what you have is to have people program a single digit on their mailbox to dial their cell ("to try to reach me on my cell press 1").
people have a great tool with the voice mail to inform people of what they are doing and when they will be back but most people don't take the 15 or 20 seconds to record their greeting in the morning and at night. Just lazy