Re:Calling a real estate robocaller back
by AnimalMuppet on Tuesday April 30, 2019 @11:24AM
(#19788765)
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.
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.