Scientific American, April 19, 1884, page 247:

Time by Telephone.
    A lawyer whose office is in the Leffingwell building stepped up to his telephone one morning, watch in hand. He did not ring or talk into the transmitter, but listened intently for several seconds with the tube at his ear, and his eyes fastened upon the face of the watch. "Just ten minutes past 11 o'clock," he remarked, as he returned the watch to his pocket and hung up the tube. "I am precisely half a minute slow."
    Only a few of the local telephone subscribers are aware of the fact that they can ascertain the correct time by simply listening at their telephone instruments. A clock apparatus has been connected at the central office with all the circuits, by which there is given at the end of each minute the hour of the day and the number of minutes past the hour. The beats signifying this are distinctly audible, although not loud enough to interfere in any degree with conversation. The attachment of this time apparatus explains the ticking sound which has mystified many persons within the last few days. The apparatus is not connected with the clock in the Yale Observatory, although it is regulated by the observatory standard.
    Manager Fairchild, of the Telephone Company, said: "The apparatus was put into the office Monday by way of experiment. It is worked by batteries, and gives the hour and the minute simultaneously over all our circuits. To ascertain the precise time it is only necessary to step to your instrument. At the end of every minute there is clicked off the hour and then the number of minutes which have elapsed since the hour was struck. If it happens to be thirteen minutes past there is one beat, then a short pause, and then three more in quick succession. The attachment works successfully, and the only question about retaining it is the cost. The expense will be about $1,000 a year, and if our subscribers are willing to pay for the accommodation they can have it."
    "But suppose the subscribers on some of the circuits are willing to pay and those upon others are not?"
    "We can render it useless on any given circuit by putting on an attachment called the confuser. This mixes up the beats so that no one can tell what they mean."--New Haven Register.