I just saw the movie Blow Out last night. I’ve got a question about a scene in which John Lithgow’s character tampers with the phone of John Travolta’s character. Stop reading now if you don’t want any spoilers for this 40-year-old film.
So in the scene in question, Travolta waiting at home almost all day for an important call from a journalist. Lithgow doesn’t want him to receive this call, and so gets into Travolta’s basement and somehow rewires Travolta’s phone line so that anyone calling into it receives a busy signal. (And the journalist indeed tries to call Travolta’s number several times, but finds that the line is always busy.) However, as far as Travolta knows, his phone is still working, because he still gets a dial tone when he lifts the receiver.
My question is whether and how such an effect would be possible in real life. I suspect that it’s not hard to rig a line so that it always rings busy for incoming calls, but wouldn’t this also suppress the dial tone and prevent outgoing calls as well?
Let’s say that Travolta lives in an apartment building (which I think was the case in the movie); then Lithgow could have unplugged two phone lines from the basement switch box—one being Travolta’s line and one being one of his neighbour’s—and then replugged Travolta’s line into the neighbour’s socket. Then Lithgow could plug something into Travolta’s old socket that would always return a busy signal to incoming callers. This way Travolta thinks his phone is working, but it’s really connected to his neighbour’s number. But doing things this way would be very risky for Lithgow, since Travolta might receive calls for his neighbour and know that his phone had been tampered with. (Or alternatively, the neighbour would learn that his own phone was defective as soon as he tried to use it and get the phone company to investigate.)
Hook up an answering machine, set to answer so quickly that Travolta never hears the first ring. Make the answering machine message a recording of a busy-signal sound. Done.
In most places, it was a recording not a busy signal, and I’m not certain that it was around 40 years ago, but punching in the keys for “Do Not Disturb” would stop inbound calls but not affect outbound. You’d also not get any indication that it was turned on, but simply a dial tone when you picked up the landline.
It wouldn’t require rewiring, as a simple butt set (or any phone) would do the trick, but I don’t remember much about that movie.
It would be similar to leaving one extension off the hook. You pick up another extension and you hear the dial tone - except a phone left off the hook after a few minutes starts giving you an insistent chirping noise instead of a dial tone, except maybe for the Hollywood exchange. You would need some gizmo to pulse a hang-up every minute or so. (Off hook would just be shorting the wires, IIRC. )
That might work, except that no answering machine was shown in the scene. All we saw was Lithgow messing around with the wires in the basement switchbox. (Also, is it even possible for answering machines to pick up the line before a phone rings?)
Features like that became available in my calling area only in the early to mid-1990s. Would a “Do Not Disturb” key-in feature have been available in Philadelphia in 1981?
The call is signaled by the pulsing of the line with a 48V (in those days) ring signal. So the same signal telling the answering machine to pick up would also make the bell ring on the phone; I presume if there were enough extensions on the line, the combined load would weaken the ring signal so much that the bells in the phones would not ring. But then, the other end would hear ringing not busy - and if Travolta picked up during such a call, he would be connected.
As DMC indicates, DND would require a phone connected to enter the codes. Touchtone? AFAIK rotary phones didn’t have “*” or “#”.
That’s a good point. Touchtones were certainly available in 1981, but there were still a ton of rotaries hanging around (remember, we didn’t own our phones back then).
For the curious, here’s the scene in question, or rather the part showing Lithgow tampering with Travolta’s switchbox and the journalist getting a busy signal:
In the movie, Travolta’s character does have a touch-tone phone. It’s placed prominently in the foreground of the shots where he’s waiting for the journalist to call.
In that little snippet, he appears to be patching in some device of unknown origin, as opposed to simply doing some rewiring. Since the device is connected directly to the line, it could do just about anything, so sure, I’d call the scene plausible.
OK, but how exactly would this little device be achieving its effect? As was mentioned upthread, busy signals were usually recordings, and that little box doesn’t look big enough to contain a tape player and a battery required to power it. Even the little pocket dictaphone I had in the mid-1990s was bigger.
Just to illustrate how dated this is, a few years ago my then 20 year old daughter was using her phone and was perplexed by a sound it made when she called the number she was trying to reach. It was a busy signal. She had never heard it before.
Sound generating chips were widely available in the early 80s and a busy signal wouldn’t be terribly hard to emulate. Also, if it were a digital phone, that device might simply have had a timer and dialed *78 (Do Not Disturb) at some predetermined time. Basically, with a black box (okay, yellow in this case), there are likely a multitude of ways in which this would have been possible at the time.
you could connect Travolta’s phone to a second unused line, and then his main line to a dummy off the hook phone (possibly the device that he plugged in).
I played around with phone/phone lines when I was a teen. I seem to remember, if you put a resistance across the two lines that was higher than the off-hook resistance of the phone (but not too high), then incoming calls would get a busy signal but the phone still worked for outgoing calls.
But as I mentioned, if the telephone company switch detects a phone off the hook not on a call for more than a few minutes, it will send a persistent pulse tone instead of a dial tone.
When we went to VoIP in our corporate office, the old touchtone phones with electro-mechanical bells stopped working, since VoIP only gave a 24V pulse, not 48. But you could hear the pulsing trying to rig the bell creating a buzz - the power was not enough to pull the hammer to the bell - unless you put the phone on its side, so gravity gave an assist. They you got a weak bell ring.
You could probably put a low-ohm resistor across the wires so that ring pulse would not be strong enough to ring the bell on the phone. Plus, presumably the resistor would not appear to the switch as “off hook” if the value was high enough. But then the caller would get perpetual ringing, not a busy signal. Picking up the phone you’d hear a dial tone and would create a busy too - the only gotcha would be if the person was calling in listening to the ring when the other person picks up the phone - they would answer the call.
There’s no simple wiring trick - So presumably that little box Lithgow wires in is complex enough to do what it was supposed to.
This scenario actually happens quite often without any outside interference or compromise. A water short on the incoming phone lines causes the higher ring voltage signal to short the two wires (ring and tip). The short is read as an “off-hook” condition by the central office equipment and so the line is switched to a busy state. The caller then hangs up and the line is free. Outgoing calls operate at a much lower voltage and there is no ring signal, so outgoing calls usually work fine.
The only thing the device needs to do is short the lines (or put a load similar to that of the telephone’s coil set) across the incoming lines when ring voltage is detected. There may be a brief sound (like a single chime) if the device is not quick enough to suppress the ring voltage…and this is almost exactly what happens when a water short is involved.
