Now that's a fascinating question. In my experience, I'd say that WU owned zero poles, but I suppose there's an exception somewhere. However, WU did own conduit-and-manhole structures as late as 1986.
In my entire career in the cable TV industry (1976-2000), I never encountered a single utility pole owned by Western Union. In fact, before I read your question, it hadn't occurred to me that WE might still own any poles.
Cable TV companies typically lease pole-attachment rights from other pole owners. Although most poles are owned by electric power or telephone companies, I've written permit applications for poles owned by municipal governments, county governments, state governments, the federal government, private landowners, educational institutions, other cable TV companies, and railroads.
But I never encountered a pole owned by WU.