I have been trying to find when encrypted radios were first used. Not something like the German Enigma that transmitted encrypted data but when a person could actually talk to another person over an encrypted radio channel.
I'm not sure if this is what you are looking for, but the A-3 Scrambler system was developed in 1939 by Bell Labs. It allowed supposedly secure communications between Roosevelt and Churchill as the Second World War broke out.
In reality the scrambler was very insecure and rather easily broken by the Germans. So between 1942-3 Bell Labs developed as a replacement the SIGSALY, which was much more technically sophisticated. Terminals of the SIGSALY system was installed around the world in Allied countries and was never broken.
The approaching war stimulated the investigation of true voice security. The BTL staff soon discovered that there were about eighty patents issued on the general topic, but analysis indicated that all of the methods were really unsatisfactory ... BTL proceeded on its own to develop this much-needed capability and was soon able to demonstrate it to the satisfaction of the Army. A U. S. Army contract was awarded in 1942 for the production of the first two systems. This system eventually came to be called SIGSALY and was first deployed in 1943.
- Boone, J. V. and R. R. Peterson, The Start of the Digital Revolution: SIGSALY Secure Digital Voice Communications in World War II, Center for Cryptologic History, National Security Agency, 2000.
The SIGSALY is held to be "the first realization of enciphered telephony" in a 1983 IEEE review.