3 ms·

Not that this explains it, but IIRC, caller ID information is sent (or was, at one time with land lines) "out of band", in a particular data format, that occurs

by cr0sh 8y ago

Not that this explains it, but IIRC, caller ID information is sent (or was, at one time with land lines) "out of band", in a particular data format, that occurs between the first and second rings. IIRC, it's done using frequencies that either can't be heard (ie, outside the freq response of the phone), or the sound is "switched off" between rings (quite possibly this, since the "ring" voltage is completely different from the "line-in-use" voltage - so this voltage change can be detected and switch the signaling as needed). At any rate, it's a known thing, and if you understand how it works, it's possible (well, again, was - for land line phones) to build a box that can inject these signals between the rings so the phone being called will display it. Normally, this is done (IIRC) by the CO, but I think if the info is already there, it doesn't override it. I might be completely wrong, though; it's been a couple of decades since I last read about how it worked; also, I have no idea how it works in cell-phone land, but likely it hasn't changed because "inertia" and having to support older land-line phones...

AnimalMuppet 8y ago
When you have an incoming call ringing on a landline, the ring signal is very much out of band - it's 48 volts, with enough current to drive a solenoid to move a mechanical hammer against a physical bell. (You do not want to be electrically in contact with a phone line when ring comes in!) The caller ID information comes as a frequency-shift-keyed (FSK) message (I think). As you said, it's between the first and second rings. But you can't hear it, not just because it's not a normal audio tone, but also because the phone is still on hook and isn't playing any audio that comes over the line. That's regular caller ID, which is type 1. Call waiting caller ID is type 2. It's also an FSK message. (That is, I'm sure that it's an FSK message. I think type 1 is also FSK, but I'm less certain of that.) This comes immediately after the call waiting "beep". It's short enough that I don't think your ear can pick it out of the transition from the beep tone to the regular conversation that the beep interrupted. I'm not sure whether you could get fake caller ID that you send through a CO. The phone line isn't "off hook" yet, so the CO isn't passing audio. (If you say "hello" into the phone while it's ringing, I don't think that audio goes down the destination phone line either.) But that's only how I think it works; I don't know.

by cr0sh 8y ago

AnimalMuppet 8y ago

When you have an incoming call ringing on a landline, the ring signal is very much out of band - it's 48 volts, with enough current to drive a solenoid to move a mechanical hammer against a physical bell. (You do not want to be electrically in contact with a phone line when ring comes in!) The caller ID information comes as a frequency-shift-keyed (FSK) message (I think). As you said, it's between the first and second rings. But you can't hear it, not just because it's not a normal audio tone, but also because the phone is still on hook and isn't playing any audio that comes over the line. That's regular caller ID, which is type 1. Call waiting caller ID is type 2. It's also an FSK message. (That is, I'm sure that it's an FSK message. I think type 1 is also FSK, but I'm less certain of that.) This comes immediately after the call waiting "beep". It's short enough that I don't think your ear can pick it out of the transition from the beep tone to the regular conversation that the beep interrupted. I'm not sure whether you could get fake caller ID that you send through a CO. The phone line isn't "off hook" yet, so the CO isn't passing audio. (If you say "hello" into the phone while it's ringing, I don't think that audio goes down the destination phone line either.) But that's only how I think it works; I don't know.