Again we are in agreement, @DelectableMe.
In fact, we have even less evidence than for your shoes for the wording in the conversation.
We have AN Wilson saying he believes it, but not saying that Charles S told him that Charles R said those words.
We have Richard Kay saying that Charles S told him he had a row with Charles R about the boys, but nothing about the wording.
He also suggested (in an interview clip) that Charles S, as a former journalist, always took notes, so no doubt would have taken notes at this time too.
Charles S, in the Times interview, denies taking any notes or telling anyone about the words used by Charles R in the telephone call:
Does he, Kuenssberg wondered, have any proof, any evidence, did he write it down or tell anyone? Well, no, not quite, but he did claim that the final words of his famous eulogy at his sister’s funeral, that “she will never be extinguished from our minds,” was “a direct rebuttal”.
How many eulogies have you heard, where the last sentence or so is a determination that the person will never be forgotten?