How do telecom/telephone scam bots work

[Dajgoro]

Hello

Recently I started getting calls from random cell numbers and when I'd answer I'd get silence.
Then I stopped answering and they would ring for a bit and stop. I borrowed another phone to call them back but all the numbers would be invalid/not in use. 
I'm also not the only one getting such calls.
Obviously its some sort of scam going on, but I'm really curious how do they do it?
How do they activate dead numbers, start a call and kill them instantly?
Are they patched to the telephone network via some modern version of the blue box?
Did they hack a telecom provider or is it some mob deal with the telecom provider so they are able to do such things on demand?
How does even today's telephone network work? Who does the line routing, is it all over IP or is it some strange digital magic relay central thing? 

[MrMobodies]

In my country it is called "Caller ID spoofing" where the real number isn't displayed.

https://en.wikipedia.org/wiki/Caller_ID_spoofing

United Kingdom
In the UK, the spoofed number is called the "presentation number". This must be either allocated to the caller, or if allocated to a third party, it is only to be used with the third party's explicit permission.[39]

It's wouldn't too bad if the real number was also displayed then the presentation number to give a clue of the real caller.

I am getting many calls like that and I try not speak first because I think there might be a possibility of capturing my voice but they could do that anyway with what may appear to be a legitimate phone call.

[Dajgoro]

So how does Caller ID spoofing work today? Are telephone networks still using modulated signals over the voice line itself, or is it a signal outside of the voice domain available to the end user?

[station240]

In India there is a VOIP provider (server farm) dedicated to routing scam calls to the rest of the world.
With Voice Over IP (VOIP), the number you've using is a software setting, some providers let you set this yourself.

The scammers simply pick a number at random, and use that.
Sometimes that number is actually in use, so some old lady or business get the abusive returned calls.

It would be really simple to block these phone calls, either by banning the VOIP provider from connecting, or by filtering the large amount of suspect traffic they create.
But the phone companies get paid for these calls, so making money is more important to them, than removing something which is annoying a large number of their customers.

[Dajgoro]

Damn scammers.
I noticed right after the calls I started getting telemarketing calls from another telekom company, probably wanting to sell me a new mobile deal, but I never picked up.
Today I got a 3 sec ringing from the UK. 

[johnh]

In India there is a VOIP provider (server farm) dedicated to routing scam calls to the rest of the world.
With Voice Over IP (VOIP), the number you've using is a software setting, some providers let you set this yourself.

The scammers simply pick a number at random, and use that.
Sometimes that number is actually in use, so some old lady or business get the abusive returned calls.

It would be really simple to block these phone calls, either by banning the VOIP provider from connecting, or by filtering the large amount of suspect traffic they create.
But the phone companies get paid for these calls, so making money is more important to them, than removing something which is annoying a large number of their customers.

I have heard from ex-colleage that Telstra is building a system to stop these scam calls. 
It will check to see if the A-party (calling) number belongs to Telstra, It it does it will block the call.

There is more going on which he didn't go into 

[Brumby]

Damn scammers.
I noticed right after the calls I started getting telemarketing calls from another telekom company, probably wanting to sell me a new mobile deal, but I never picked up.
Today I got a 3 sec ringing from the UK. 

There is some valuable information they can get from making thousands of random calls...
1. The call doesn't get routed - they know to mark that number as of no value
2. The call reaches a handset - they know the number is active. Potentially valuable!
3. The call is answered - they know the number is monitored.
  (a) answered by fax machine tones - might be worth something
  (b) answered by human - JACKPOT!  Definitely worth selling this information off!!!! ... and why you got telemarketing calls.

[Gyro]

Apparently it's going to stop in the UK...

Major phone networks have agreed to automatically block almost all internet calls coming from abroad if they pretend to be from UK numbers, Ofcom has confirmed.

https://www.bbc.co.uk/news/business-59032795

I'll believe that when I ("almost") see it! 

[Brumby]

A couple of hours ago I got a call from someone who says they had a missed call from my mobile.  Funny ... since I haven't made ANY phone calls at all today.

I suspect my number has been used in a spoof.   

Thankfully, I've not had any further such calls - so maybe mine was just random - but I'm not relaxing my guard just yet.