Recently, we wrote that VoIP could be more secure and just as reliable as the public switched telephone network, and readers have asked us to elaborate.Much of the PSTN\u2019s security is built around physical security. For example, we lock our building\u2019s \u201cphone closet.\u201d Shared facilities like the phone company central offices and cable vaults have high levels of physical security. We also protect information about how physical connections are made. For example, the numbers assigned to the cable pairs that connect your PBX trunk to the central office are not publicly available.Despite these precautions, there are still holes in PSTN security that make it very easy for those with criminal intent to intercept your calls. A screwdriver can pop the lock on a telco closet. Telco closet wiring is usually labeled so that a cable pair can be easily identified by telco maintenance. The same marker can identify your assigned phone number. And a handset equipped with alligator clips can be used to listen in on you calls, provided the calls are sent to the PSTN on an analog carrier channel. Digital carrier channels offer a bit more security, as they require a decoder to convert the digital signal back to a sound the human ear can translate.Lesson one: Even standard physical security measures can be easily overcome, either in the PSTN or on a VoIP network. Lesson one corollary: Conversion to a digital carrier complicates interception by one more one step.Next time: Security and the signaling infrastructure.