All speculation on your part as there is no evidence either way to prove or disprove a telephone call was made or not made. You can offer this up as speculation, but not as proven evidence.
Evidence of whether a call actually occurred is:
1) whether Nevill had motive to call Jeremy
2) whether Nevill had the opportunity to call Jeremy
3) whether Jeremy's actions were consistent with someone receiving such a call
4) any other evidence that speaks to the issue such as Julie's testimony Jeremy told her in advance as part of his plan he was making such a call up to support his claims
Did Nevill have motive to call Jeremy? No. The evidence proves Sheila can't have shot anyone but rather someone else shot her and everyone else so that means he had no reason to call Jeremy and claim she had a gun.
Even if Sheila did have a gun there would be no reason for Nevill to call Jeremy. Jeremy could not have calmed her down and had no ability to physically disarm her from far away. Nevill had the ability to disarm her in contrast and need plus was stronger than her or Jeremy. He also had access to guns, kinvies and other wepaons he coudl have used to confront her. If too scared to confront her he would have dialed 999 not calle dhis son's answering machine. That is another problem Jeremy would not have been able to reach the phone even if he had woken up, the answering machien woudl have picked up.
Nevill not only had no motive to call Jeremy and tell him that Sheila had a gun he also had no opportunity. The killer walked into the bedroom and shot both Nevill and June before they could do anything. There was no phone so no way Nevill could have called Jeremy before any shooting started.
Jeremy's actions are totally inconsistent with someone receiving such a clal and he evne had to lie and say he claled police before Julie to try to pretend his actions were somewhat consistent with how someone in his place would act.
Everything supports Julie's claim that he told her in advance he planned to pretend he received a call from WHF.
While you say this is not evidence rational people who actually understand what evidence is disagree.