In Russia, when you cannot get through a certain number, you say that the number is engaged ‘like at the Smolny’. The Smolny Palace was built by Giacomo Quarenghi in 1806-08 to house the Smolny Institute for Noble Maidens, Russia’s first institution for women’s education. However, in 1917 the Smolny Institute was moved out of Petersburg, and between October 1917 and March 1918 the building served as the headquarters for the Bolshevik Government. Vladimir Lenin resided here, and several revolutionary decrees got passed here in November 1917, including the Decree on Peace and Decree on Land. Naturally, the phone line must have been engaged most of the time, due to a high activity of the Bolshevik government, and so the abovementioned expression originated.
The Museum of Contemporary History in Moscow’s Tverskaya St. has tweeted a picture of the phone that was used by the Military Revolutionary Committee in Vassilevsky Island to keep in touch with the Bolshevik headquarters at the Smolny.
— Музей совр. истории (@SovrMuse) August 24, 2017