What is that annoying sound?

Question

when you dial a fax machine # on acident, what in the world is making that high pitched wailing?

Answer 1

It's a formal process by which the analog signals are synchronized and thus able to be translated into other forms.

Answer 2

You are hearing the reverb through the wires. Every modem makes that sound when you are trying to connect to it. It happens in most telephones too only we do not hear it as much.