If he called GH from WHF, depressed the button and just left the handset on the counter, it would be off the hook but the call to WHF would be terminated. The operator could still break into the line. Even if it had been answered from the other end and left connected, the operator could still listen in. I'm not sure what point your're trying to make. Why would he need to do his homework? That's how phones worked then, he was surely familiar with the procedure. 
The scenarios and the mechanical processes are entirely different:
a) NB phoning GH from WHF. JB receiving call from WHF. NB leaving handset on worktop thus not ending call.
b) JB phoning answering machine at GH. Answer machine kicks in effectively being answered. JB depressing cradle at WHF and effectively terminating the call. This was the scenario you gave:
http://jeremybamberforum.co.uk/index.php/topic,6179.msg274576.html#msg274576Why would JB be familiar with the capability of the operator in the 1980's? I certainly would not have been.
Last week you doubted JB would be offay with the fact that Witham police station covered WHF and now, this week, you think he should be offay with operator capability. Lol.
Are you able to provide your sources?