Scientific American, September, 1925, page 174:

How  the  Parisian  Enjoys  Opera  at  Home

Telephone  Subscribers  Enjoy  a  Unique  Service  From  a  Central  Switchboard  By  Means  of  an  Elaborate  System  of  Amplification

By  Frederic  M.  Delano
aris and Munich are perhaps the only two cities in the world that have a regular service of grand opera to telephone subscribers; and of the two, Paris is unique in having installed a system at the central bureau which permits listeners to hear the music on the loud-speaker. Inside the Theatre Trianon-Lyrique
    The service is, of course, not open to all regular telephone subscribers, being operated by a private company, the Compagnie du Theatrophone, which has contracts with both the theatres and the telephone company to handle the distribution of music and other entertainment over the telephone lines. Among the theatres in the agreement are the Opera, the Opera-Comique, the Trianon-Lyrique, and the Comédie Française. These four are perhaps the most famous in the French capital; and are the most popular with that large group of Parisians who usually desire to remain quietly at home to listen to good music or dramatic performances.

Music  Over  Regular Telephone  Lines

    The Theatrophone company has a private central switchboard, into which pass the trunk lines from the different theatres, and from which pass the lines which carry the music to their many subscribers. On the stage of the theatre (usually behind the footlights) are microphones--sometimes two, sometimes more. The voices of the actors or singers are transmitted by these microphones to the first stage of amplification.
    In a special room in the cellar of the theatre, well away from the possible disturbing elements of the action on the stage, are the amplifying units. Two sets of batteries, one for the microphones and one for the amplifiers, are ranged around the room. The amplifiers are enclosed in large cabinets, two complete units of three tubes being used. They can be either operated separately or together; or in case of emergency, the second amplifier can replace the first.
    These amplifiers are very similar to those used in radio, although their construction was specially worked out for the theatrophone. Great care has been taken to avoid any outside sounds from reaching them, and thus creating a line disturbance. The amplifiers are enclosed in large cabinets with double sides of padded cork and the amplifying tubes are set in sockets held in elastic supports.
    The impulses from the microphones, even after they have passed through this first series of amplifiers, are only about as strong as the average telephone voice. The impulses are now carried directly to the "theatrophonic central exchange," where they are again amplified, divided, and distributed over the main telephone lines to the subscribers. Theatrophone Central Switchboard    At the "theatrophonic central" it is picked up by another series of amplifiers, the usual three-electrode tubes being used; and passed from there after being stepped-up to loudspeaker volume, to the "distributor". This is a sort of amplifier which has as many tubes as there are subscribers.
    From the central station the current passes to the jacks of the distribution boards, by which it is sent out to the various city telephone exchanges. The theatrophonic lines are placed in the central stations so that they are automatically protected and insulated from multiple communications, such as bells, or other possible sources of interference. From the P. T. T. (Postes, Télégraphs, et Téléphones) central stations, the communication to the theatrophone subscriber is made over the regular telephone lines.

Subscriber  Has  Choice  of  Theatres

    The operation of the theatrophonic central board is simple, and suggests a regular telephone exchange. The operators come in at 7:00 P. M., take their places before the switchboards, each board being connected to one or more telephone central stations. At the top of each board are the subscribers' lines, with plugs. The board has jacks connected to the distributors, and all that is necessary is to plug in the jack of the subscribers with the plug or the particular theatre he wants.
    The operators have before them a list of subscribers divided into groups. For instance, in making a connection with the subscribers on the "Wagram" exchange, the theatrophone operator opens her key communicating with the city "Wagram" exchange operator and calls off her list of subscribers. In this way she covers all the telephone exchanges, making the necessary connections. Then the city operator calls the subscribers on her exchange, and notifies them that they are now in touch with the theatrophone.
    In the meantime the theatrophone operators have plugged in to the theatre. At 7:45 the distributor tubes light up, and the amplifiers are put into service. The primary check-up of the proper tuning and the noise of the audiences going into the theatres, as well as the orchestras tuning up, is done at the central office by a theatrophone engineer.

No  Interference  With  Regular  Service

    The switchboards are covered with an intricate network of wire, each leading to a jack over a small signal lamp. At times, one of these small lamps will light up, indicating that subscriber wishes to call the theatrophone exchange, either to change from one theatre to another, or to break the connection so that he may call an outside number in case of an emergency.
    If, on the other hand, someone wishes to call the subscriber from an outside telephone, the call is passed to the theatrophone operator, who informs him that the subscriber is listening to the theatre. If the outside call says it is really a necessary and important call, the connection with the theatre is broken, and the incoming call put through. The theatre connection can be restored after the conversation has been completed.
    The loudspeaker attachment, with the amplifications in the exchange, instead of at the houses of the subscribers, is the important and novel step that has been made in this work. The French were the first to perfect and install such a system, and they are now the first to perfect this distance amplification, which is making the theatrophone a far more popular source of amusement, due to the wider possibilities of entertaining large parties.