It looks like a telephonemetre stop watch. I had a quick search on google images using zenith stopwatch, and quite a few came up looking similar although not 100% the same.
Yes, a telephone call timer. Zenith made many of them, and they appear on the market with some regularity.
According to what I found on google, they were used by telephone swithboard operators around the turn of the 1900s. At that time in America, for example there were only 5 mill phones in the US so it was probbaly not a huge job to moniter all the calls and work out the charges, or make sure that no one was getting more time than they were paying for. According to the patent drawings they had mechanical movements so I guess they wound them up like regular clocks. According to another article I found, they were still in use in the 1940s. This one looks like a later one as it doesn't have the patent number on it like earlier ones did.