The wrong letter

The telephone bell rang on the table of Superintendent Sinclair at Scotland Yard. He was a busy man, and had given orders that he was not to be disturbed except on matters important.
Putting down a paper he had been reading, he picked up the receiver. A woman’s voice spoke.
“Is that Scotland Yard?”
“Yes, yes,” he said impatiently, “Superintendent Sinclair speaking, what is it?”
“Listen carefully,” said the voice. “The Home Secretary has been murdered at his own house, it would be as well if you would come at once. Have you got that? Just repeat.”
Even Sinclair, the coolest head in the service, was staggered for a moment. There was not a trace of hurry or emotion in the voice. It might have been inviting him to tea. Before he could collect himself, the voice began again.
“I will repeat,” and the same impassive message came through with the concluding words, “Have you got that?”
Sinclair pulled himself together.