Can callee determine who has called and gotten a busy signal?

Factual Questions
1

Here’s the background:

My city is receiving some doses of COVID vaccine. They set up a call center so that eligible residents could make an appointment to get the vaccine. The call center opened this morning.

My husband and I started calling right at the time the call center opened. I tried several times and always got a busy signal. Husband got through, but only to a phone queue – one of those “Press 1 and stay on the line. Due to high demand you may hear this message again.” After 2 hours of this, his call was dropped.

According to the city, a robot calling system inundated the call center with over 65,000 calls and calls never got through to call takers. They however obtained a list of callers and would call them in the order in which they were received.

Early this afternoon, a call from the city came in on our landline, and husband was able to make an appointment. I was using the landline, and my call never went through – I just got a busy signal on every attempt. Later, they called my husband’s cellphone.

I can understand them calling the cellphone number, as his call got through to the endless call queue. But how did they know to call on the landline?

2

My experience with software like Twilio is, if a call gets through to your cloud at all, then not only do you have a record of who called you, you determine what (if anything!) they subsequently hear, whether it be a busy signal, a ring tone, a robotic voice, top of the pops, whatever.

3

Interesting. Thanks!

I had always assumed, and (based on what others had to say when I told them about this) that the busy signal was sent by the call receiver’s phone company and that the call never got to the call receiver’s equipment.

Maybe this is how it worked years ago. I know there used to be two kinds of busy signal – the one you typically hear, and a faster one which was supposed to mean that the call never got to the point where it could be determined that the receiver’s equipment was “busy.”

(Maybe I didn’t say that too clearly. I mean that if something like a storm took down all the phones in an area, callers would get the faster busy signal.)