What do you mean 'keep the line open'? The line wasn't 'open' until the operator connected to it much later.
All I can go off is the note in Court of Appeal Judgement
"68. There was no evidence of telephone billing information of the sort which would be available
these days. There was, however, expert evidence called as to the effect of a telephone call
having been made from White House Farm to Goldhanger which was then abandoned by the
caller with the receiver being left off the cradle, as claimed by the appellant. If such a
sequence had occurred, the telephone link would have remained open either until the
handset at White House Farm was replaced or until the handset at Goldhanger had been
replaced and left in position for a period which could vary from 1 to 2 minutes, when an
automatic interruption of the link would take place. Until one or other of these events, the
appellant would have been unable to make any call from the Goldhanger telephone.
The words remain open are in there. "If such a
sequence had occurred,
the telephone link would have remained open either until the
handset at White House Farm was replaced or until the handset at Goldhanger had been
replaced and left in position for a period which could vary from 1 to 2 minutes, when an
automatic interruption of the link would take place. Until one or other of these events, the
appellant would have been unable to make any call from the Goldhanger telephone."
So that fits perfectly with what Jeremy said, in that he rang back and got an engaged signal, not a fault signal.
https://murderpedia.org/male.B/images/bamber_jeremy/20011745.pdf page 11.