Here's a pieced-together social and technical history of the Kremlin-White House hotline, a fascinating story of crypto, diplomacy and wicked hardware:
The method to be used was one-time tape. Section 4 of the annex to the memorandum stated: "The USSR shall provide for preparation and delivery of keying tapes to the terminal point of the link in the United States for reception of messages from the USSR. The United States shall provide for the preparation and delivery of keying tapes to the terminal point of the link in the USSR for reception of messages from the United States. Delivery of prepared keying tapes to the terminal points of the link shall be effected through the Embassy of the USSR in Washington (for the terminal of the link in the USSR) and through the Embassy of the United States in Moscow (for the terminal of the link in the United States).
For its one-time tape hardware, the US would employ the ETCRRM II, or Electronic Teleprinter Cryptographic Regenerative Repeater Mixer II. One of many 'one-time' tape mechanisms sold by commercial firms, it was produced and sold for about $1,000 by Standard Telefon Kabelfabrik of Oslo, the Norwegian subsidiary of International Telephone and Telegraph Corporation, the same company which installed the American terminal in the National Military Command Center deep within the Pentagon. It has four teleprinters — two with English alphabet and two with Russian — and four associated ETCRRM II's . In Moscow, the terminus was installed in the Kremlin, near the office of the Premier".
The Washington to London portion of the link was carried over the TAT-1 (Transatlantic No. 1), the first submarine transatlantic telephone cable system. It was laid between Gallanach Bay, near Oban, Scotland and Clarenville, Newfoundland between 1955 and 1956 and was inaugurated on September 25, 1956.
(via Beyond the Beyond)