The Southern Massachusetts Telephone Company had subscribers in Cape Cod. After their beginning in 1878 the company gradually sold their exchanges one-by-one to NET&T Co. until they were fully absorbed by Ma Bell in 1938. I do not know when the latter purchased the Dennis, MA subscribers however if not by the time this image was taken then the sign you see on this storefront is a So. Mass. Tel. Co. one. These signs are very rare and are somewhat similar to the NET&T Co. ones of the time.