The Telephony Hall features a collection of telephonic devices and switchboards from 1882 to 1924. One item on display, from the model invented by Alexander Graham Bell in 1876, is a primitive Bell telephone, a device consisting of two pieces of wood and a separate receiver. The next group of devices are what is known as local-battery devices, composed of a dry-cell battery and a switch to activate the bell that announces an incoming call, such as the Mix & Genest model, a cabinet phone that belonged to Queen Maria Christina of Hapsburg-Lorraine, or the luxury Ericsson phone from the office of Calvo Sotelo in the Ministry of the Treasury. In later models, known as "magneto" phones, the electric current was triggered by a lever. An example is the luxury Ericsson brand magnetic phone. After Hayes' invention in 1892, local batteries were replaced with a central battery, which powered all the devices connected to it. The wall phone with a transmitter mouthpiece and the intercom manufactured by the Spanish Tele-Ibérica company employ this system.
The operation of connecting users to one another was carried out through switchboards or manual exchanges. At first, these switchboards were modelled following the Swiss design used in telegraphy, but when the number of customers grew, the jack system was adopted, with pairs of cord plugs allowing a larger number of connections.
Finally, automatic service replaced this type of switchboard and telephone devices were modified with Strowger's invention, which gave them an alphanumeric rotary wheel that allowed users to connect directly, eliminating the need for operators and making conversations more private.
On display in the telephony hall are several desk-model and local-battery switchboards for thirty, fifty and two hundred users, which date back to the late 19th and early 20th centuries and were obtained from the Ministry of Public Works, the Senate and the Almeria Provincial Office of Communications.
Completing the exhibit are two telephone switchboards that were part of the equipment at the Moncloa Palace Telegraph Office from 1980 to 2001.