I don’t know how long B. & H. Electric has been in Prospect Heights, but the NE phone exchange came into use in 1930, when the New York Telephone Company greatly expanded its dialing system.
SA also came into existence in 1930; it covered West Harlem. This rusty relic, advertising another electric company, is still hanging on outside an apartment building in the West 150s.
SA stood for Sacramento. But why Sacramento?
For those perplexed by these and other mysteries of old letter phone prefixes, here’s an exhaustive website that can shed a little light.