Edificio Unión Telefónica, Eclectic architecture building in Monserrat, Buenos Aires, Argentina.

The Edificio Unión Telefónica, originally called Edificio Defensa, stands at Defensa 143 and 159 in Monserrat. It has a ground floor and 11 upper floors, and its H-shaped plan forms three bodies joined at the centre, leaving four light wells. Two entrances are carved with shell-shaped stone ornaments, and the narrow street makes the full facade hard to see.

The United River Plate Telephone Company, formed in 1882 from three earlier Argentine telephone firms, commissioned this headquarters from Héctor Calvo, Arnoldo Jacobs and Rafael Giménez. Construction began in 1928 on the site of an Obligado family house from the late 18th century, and the building opened the following year. Nationalised under Juan Domingo Perón, it passed to the state in 1948 and was run by ENTel from 1956; Telefónica de Argentina took it over in the 1990s privatisation.

The building's name comes from the Compañía Unión Telefónica, formed in 1882 by merging Argentina's first telephone firms: the French Société du Pantéléphone L. de Locht et Cie., the English Compañía de Teléfono Gower-Bell and the American Compañía Telefónica del Río de La Plata. The company itself called it Edificio Defensa, after the street. Floors were assigned by function: the Avenida automatic exchange on the first floor, the general administration on the seventh, the staff dining room on the eighth and the print shop on the roof.

The building occupies the full width of Defensa Street at numbers 143 and 159, a few metres from Plaza de Mayo, but the street is so narrow that the whole facade cannot be seen from the pavement. It has two entrances, and the H-shaped plan leaves four open light wells between the three sections. It remains an administrative office of Telefónica de Argentina, so the interior is not open to visitors.

The structure spans three distinct sections across the entire plot, with specialized functional areas within each. A rooftop printing facility once produced materials for internal operations.