The noise you hear when you pick up a telephone to tell you it is working and waiting for you to dial a number.
It is a dial tone ( like Morse Code) and functions as the carrier wave for the phone call, once ( dialed up) the message traffic rides on the carrier. ( not necessarily Flat on top- pun intended with...
ring ring xx
you my face that problem because of network or have access to internet.
Telephone Line - ELO
you might want to be a bit more specific.