How did Kemp's maid describe the ringing of the door bell?

No one was there, thus the maid described it as a "runway ring" or a prank.

He tried to resume his work, failed, got up, went downstairs from his study to the landing, rang, and called over the balustrade to the housemaid as she appeared in the hall below. "Was that a letter?" he asked.

"Only a runaway ring, sir," she answered.