Isnt it just common sense? Jb alleges he gets a call from dad. But there is no proof of one. So how did jb know his family were dead unless he was there when they died.
Some alternative slants:
(1) There is no proof a call
didn't take place. In which case Stan has some explaining to do regarding why he told Mugford that a call definitely
didn't take place.
(2) There
is proof of metered a cheap rate call, most likely to be the call in question.
(3) What on earth was he doing executing an audacious plan to carry out five murders, the success of which would be dependent upon a call that could either be proved as never having taken place or at the very least be inconclusive? It's hardly likely is it?