telephones

Can anyone explain how the telephones worked? It seemed that whenever Colin called Hazel her phone did not ring in a normal way? it made a quiet sound and then she would lift the receiver and dial a short number and was then connected to him.
I briefly thought it was like a code they arranged, like he would ring, hang up after one ring and she would then call him back if she could talk, but the first time he called her at night, she seemed surprised at his call, so therefore it couldn't have been a predetermined code.