March 17, 2013

IN THE 21ST CENTURY, it’s becoming impossible to hang up the phone.

But actually, hanging up even an old-style phone didn’t physically end the connection, but just told the central office switch to do so. And eavesdroppers at the phone company switch could tell it to ignore the hook position, allowing them to listen to whatever the phone could pick up.

UPDATE: A couple of readers say I’m wrong about this, and perhaps I am, but that’s my clear recollection. I also remember as a kid hanging up the phone and having it not disconnect the call on a few occasions.

ANOTHER UPDATE: Reader Ron Muir writes:

On the subject of PSTN telephones and the network, as an “old” telephone guy. The systems allow the person originating call (Call Control) to terminate it by hanging up (pushing down the switch hook), the person receiving the call can hang up (push down the switch hook) but the call will not terminate until the caller hangs up. The new digital systems with SS7, change the dynamic slightly and make it an option for the originating exchange carrier to recognize the receiving caller to hang up (effect Call Control).

PSTN means “Public Switched Telephone Network.” Meanwhile, reader Douglas McKinnie emails:

What you have described is two unrelated issues –
There are times when the central office doesn’t register the change in impedance that indicates you have hung-up, and so the call remains connected even though your instrument is switched off. Picking up the handset again reveals that the line and call are still active.

The second issue is the eavesdropping bit. The old USA Model # 500 telephones typically had 4-wire (2 line) cords and 4-pole hang-up switches. The way they were designed, hanging up would switch OFF the handset and allow the ringer to remain connected. The other wires had use if a second line was required, for signalling functions such as the message light on hotel phones, and for certain functions of party lines so that the right house would ring.

Now, the ringer was to some extent microphonic, so it is in theory possible that someone at the CO could listen to conversations that way. They would have to be loud enough to move the ringer coil in relation to its unintentionally magnetized core.

The more likely issue is that somewhere along the line someone “happened to” just connect the 4-pole double-throw switch such that hanging up disconnected the handset from the normal two wires of the telephone line, and connected them to the other two unused wires. Then, anyone with access to the building telephone wiring could listen across those two wires to hear conversation through the on-hook telephone handset.

I recall reading about one government agency that decided to check, and found most of their phones wired that way.

Hmm.