In 1918, most telephone exchanges had not yet automated and still used human operators to handle calls. And "party lines" were common among rural exchanges, where numerous subscribers shared a common telephone wire, and knew when a call was for them according to the number of times the phone rang. In normal circumstances you were not supposed to eavesdrop on other conversations on the line, but the "Telephone Announcement Service" used a special number of rings to let subscribers know when a general message was being set out.

Telephone Engineer, February, 1918, page 80:

Indiana  Company  Gives  News  Service

    On February 1 the Greenfield, Ind., Telephone Company started what is called a farm line announcement service, which consists of announcing over each farm or rural line, the weather forecast or report, the hog and cattle market reports and the correct time of day.
    The operation of this service is as follows:
    The telephone operator, using but one line at a time gives ten or more short rings on the line, and to all subscribers on the line this is known to be a general ring for the purpose of calling everyone to their telephone. Upon receiving such a call everyone takes down his receiver and listens very eagerly for the daily markets or other current news, such as special sales of local merchants or prices which Jones, Smith or Brown may be paying for poultry and eggs, farmers' closing out sales, etc., etc.
    The merchant who is conducting a special sale or who is opening up a new season's goods will find "Telephone Announcement Service" an effective way of calling the attention of all rural telephone users to what he has to offer.
    The farmer who is planning a closing out sale will find that a "Telephone Announcement" will bring a large crowd to attend his sale. Every auctioneer will readily see the value of this service.
    This form of advertising will help the local business which some glittering display advertisement is now pulling to the cities.
    This service is new to that part of the country and there is no doubt but that the business men of Greenfield and vicinity will take advantage of this new and novel way of reaching the farmer and rural route trade.