Please reword this sentence. It seems a little awkward.

I have seen more and more signs like this at businesses in recent years.  There is no front desk, no receptionist, just a little room with a phone and a list of extensions.  You have to know who you want to see, find their name, and then dial the corresponding number.  If that is what you mean to convey, here's one way.

Please use the wall phone to contact the employee you are visiting.  Dial the extension number as shown on the employee list.