AT&T's very rare Security-Plus Telephone

Since 1987, AT&T advertised a new secure telephone that operated according to the STU-III standard of the NSA. This was the Security-Plus Telephone , which is probably one of the rarest devices from the STU-III family. Already in 1992, AT&T replaced the Security-Plus Telephone by a completely different model that became more commonly used. Advertisement from 1987 for AT&T's Security-Plus Telephone…