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Had yet another #robocall #scamcall on my #AAISP SIP line this morning. I've fallen down a rabbit hole: how can I block these calls?
The spectrogram is very distinctive. Call pickup is followed by an unnaturally low noise level for a couple of seconds.
Then there's a blip.
Then the noise level rockets.
Sharp tones at 50Hz and 100Hz and harmonics pop up. That's AC mains noise. But what about that blip?
Seems it's 100ms long, starting at around 440 Hz (the A440 musical pitch reference, A4 on a piano).
Idea: pick up after a few rings and start analysing the audio. If the background level is too low, play a voice saying "Hello?" and see if a chirp comes back.
If the other end chirps and the noise level rockets: flag the call as spam and route it to a hold queue. The hold queue plays random "Oh that sounds interesting" sound clips until the scammer hangs up.
If no chirp: play the BT call-waiting notice ("Please hold. The person you are calling knows you are waiting") and ring the extensions normally.
@philpem If its a SIP line, run a local PBX and route it through that, then use a geographic IP allocation database to get the block that address belongs to and add it to your iptables (much more effective than blocking indivudual addresses).
@RavenLuni Sadly I'm fairly sure they're not coming in over IP. Sadly @aaisp don't seem to offer many call-blocking features, I'm having to do it on Asterisk.
@philpem sort of around this, if I get a call from a number I don’t recognise, I answer but don’t say anything. Autodiallers will hang up assuming something is wrong or there’s no human; people will say, “hello?” after a couple of seconds