I wasn't aware the expert telephone evidence at trial was a prosecution point? It certainly does not feature in the prosecution case outlined in the CoA 2002 document.
You have literally quoted the CoA recounting the evidence produced at trial!
IIRC, JB claimed he tried to call his father straight back but got an engaged tone and the prosecution called the expert witness to show that Bamber's account could not be right, he would have still been connected to WHF unless he waited 1-2 minutes.
While not a prosecution point on its own, the intended effect was to show the jury that Bamber's account of his father calling was at best unreliable and at worst, a pack of lies - which was very much a prosecution point.