Call Progress Tone Puns

So here goes. I've been living in UK (London) for 2 years. I've lived in 2 different flats during that time. And from time to time I just hear these noises (the ones you hear when you're calling someone before they pick up. Call waiting tones?) I can hear them so clearly, but then when they stop I don't hear anyone talking or anything. They just stop. For the first few months I thought that they were just in my head, or maybe my neighbour was using some sort of hearing aid. But then I moved places and I still hear them! Is that a common thing? Do you get that too?

https://twitter.com/PeteThamel/status/1419482310588739588?s=19

I’m sorry if this question may sound silly, but I haven’t been able to acquire a clear answer from research.

For example, let’s say I have a POTS line coming in from the telco provider, just a standard 2 conductor wire terminated to a 66 block. I then terminate the premise cabling to that line to be used for a fire or security panel.

The panel at this point can receive dial tone just fine, it can dial out. However, what happens if I try calling the number that’s assigned to this line? Will it interrupt the service or does a device that’s receiving the dial tone have to “pick up” the incoming phone call?

Thank you!