Someone was looking for the book with the call number PL794.K613 and could not locate it upstairs. No one answered the Stacks Phone, what should I do?

I checked the call number and according to the catalog there should have been a copy upstairs. If you can't reach someone in Stacks the next thing you should do is use the "Request Item" feature. What will happen is someone will search for the book for that patron. When they find it they will send them an email that is is here and ready for pick-up on the Lower Level hold shelf.
Always check the record when someone says the book is upstairs. Sometimes the book is actually in a different location like Reference, Reserves, etc.