You’ve heard the sentence used in books and movies many times. “Is this line secure?” or “get me a secure line.” You understand that a secure line means a phone call where no one can listen in on the conversation, but how does that work? What does a secure phone line really mean?

What Exactly Is a Secure Phone Line?

A secure phone line is one where end-to-end encryption ensures the voice security of the call participants. Only the people on a call are privy to the conversation. An unknown third party cannot listen in on the conversation or, better still, does not even know of its existence.

The point of encryption is to prevent Man-in-the-Middle (MITM) attacks on the call; in this case, phone tapping.

Early forms of phone tapping actually involved using special equipment, such as an induction coil or a beige box, to tap a phone line. But phone tapping techniques have evolved. There are now several ways and advanced equipment a well-funded, motivated entity may use to listen in on a conversation of interest. Common tools include IMSI catchers, special satellites, or software.

What Makes a Phone Line Secure?

The workings of secure phones are public information—or at least, part of it is public information. You would need a device equipped with a voice encryption system. The security of the system primarily rests in its encryption and key management. The call participants must also follow a special protocol to secure the line.

The first generation of secure phones was the STU, short for Secure Telephone Unit. STUs became obsolete in the 1990s and were replaced with Secure Terminal Equipment (STE) and devices that use the Secure Communications Interoperability Protocol (SCIP). STEs have a special cryptographic card, like an ExpressCard or a PC Card. This card contains a cipher and the keys used for the encryption.

These devices are made by the R&D units of the military or telecom companies—AT&T once developed secure phones too. So, you would need to find and contact a phone company or vendor willing to sell proprietary devices to a regular person. That’s reaching, but fingers crossed.

How to Get a Secure Line

STE and STU are beyond the reach of regular people. Nevertheless, there is still a consumer option: Voice Over IP (VOIP) technology. VoIP technology makes eavesdropping or phone tapping difficult. Popular VoIP service providers include Skype Call, Google Voice, and Zadarma.

The aforementioned VoIP solutions don’t require you to get a special device. They work just fine on your smartphone. You’ll still need to download an app and get a service plan, though. There are also VoIP solutions where you use special hardware (adapter or phone), e.g., Vonage.

Alternatively, you may consider social apps that foster encrypted communications. You could, for instance, look into how Signal works, or similarly a close alternative like Wire. In addition to removing security concerns that plague much of telecom, VoIP is cheaper too. However, low price is not everything. You should consider what switching to VoIP will mean for you.

Secure Phone Lines: Are They Worth the Bother and Expense?

The security of phone calls is not guaranteed. However, you don’t have to get special equipment to secure your voice calls. Military and government equipment is out of reach for most consumers. There is the problem of compatibility—the call protocols on STU and STE may be different for the receiving device. Nevertheless, these issues don’t mean you have to give up on your phone privacy. There are commercial options that can meet your privacy and security needs.