Back in 1915, full-length films were often no more than half an hour. There were exceptions but in general the film industry and standards were evolving and the full-length film we think of today was more common in the 1920s. So, at only half an hour, "Crossed Wires" was about as long as most films were...though there were exceptions (such as "Birth of a Nation").
As for the plot, it's utterly ridiculous--the sort of thing that is just too impossible to believe. But, back in 1915 this sort of melodrama was easier for the public to accept and you can't completely judge the film by today's standards.
The story begins with the elderly Mrs. Grayson writing her will. Her conniving maid knows she'll be coming into the money and is planning on expediting matters! But before she does it, Mrs. Grayson's nephew warns her that he doesn't trust the maid...and Grayson chucks the nephew and niece out of the house. Soon after, the maid poisons her and locks her in a room which happens to have a telephone...and what happens next is too hard to imagine possible. I'd say more but don't want to spoil it.
The film has very good and the production values of this Thanhauser movie are lovely for the era. Worth watching for silent film nuts (like me) but otherwise you might want to try a few other silents first.