Final answer:
A Secure Telephone Unit (STU) or Secure Terminal Equipment (STE) on the PSTN can make secure calls with the presence of a crypto card, which activates encryption features to protect against eavesdropping.
Step-by-step explanation:
The device in question on the Public Switched Telephone Network (PSTN) that can place a secure call when a crypto card is present is known as a Secure Telephone Unit (STU) or Secure Terminal Equipment (STE). These devices are capable of encrypting voice communications to ensure security. When a crypto card, which typically contains cryptographic keys and algorithms, is inserted into the device, it activates the encryption features. The call is then encrypted, making it secure against eavesdropping. The process typically involves the authentication of both calling parties and the establishment of a secure encrypted channel over which voice data is transmitted.
The use of such technology is often critical in military, diplomatic, and certain business contexts where sensitive information needs to be discussed over the telephone. By utilizing cryptographic techniques, users of STUs or STEs can communicate with confidence that their conversations are protected from unauthorized interception. These secure calls are part of a broader set of measures often employed to protect information in transit, commonly referred to as communications security (COMSEC).