Lots of good study material on Nabokov’s shortest short story, Signs and Symbols.
I personally think Nabokov’s general idea was this: The young man is considered insane because he fears inanimate objects, but by the end of the story, we fear the telephone because of the potential message it might convey to the elderly couple. There is apparently much more to it, as can be seen in the following essays by Alexander Dolinin and Roy Johnson:
“Contrary to the prevailing line of criticism, I take Nabokov at his word and argue in this article that “Signs and Symbols,” like “The Vane Sisters,” is constructed according to a specific “system” of concealment and does contain a neat soluble riddle whose function is similar to the acrostic puzzle in the later story.” – Alexander Dolinin, The Signs and Symbols in Nabokov’s “Signs and Symbols” from the Nabakov Studies section of The Nabakov Society.
And from Roy Johnson’s tutorial on the story,
“We are given no further information, but it is impossible to escape the implication that the call is from the hospital with news of another and this time successful suicide attempt. For if it were another wrong number there would be no relation at all between these calls and the remainder of the story.
“It is not possible to ‘prove’ that this is the case, but it is quite obvious that Nabokov is inviting the reader to supply the missing explanation. Thus the old man and his wife do have a further blow waiting for them, and the second link between the two subjects is made – in the reader’s mind.”