There are several reasons/facts why Neville would not call Jeremy -
Jeremy may not answer. Regardless of whether he heard the phone ringing or not.
Neville may not have time. Sheila was going crazy. There were five people inside WHF who were the priority.
He may not answer for a long time as was 'sleeping like a log'.
Neville may only get the answering machine.
Neville will assume the answering machine is switched on. Making it impossible for Jeremy to answer the phone within 3 - 8 rings as he was 'sleeping like a log'.
Jeremy did not like Sheila or understand her illness. So would just make things worse when trying to talk her down.
Sheila did not like Jeremy.
Another adult, June was available at WHF.
The rifle was not powerful, being used for vermin. Neville would know this. So Neville would tackle Sheila prior to shots being fired.
Sheila had 'limited'/ no experience with guns. So Neville would tackle Sheila prior to shots being fired.
Neville could restrain Sheila easily. If fully or even partially fit.
Neville and June together could restrain Sheila easily. If fully or even partially fit.
Neville could call the police. Something Jeremy is saying he did.
Jeremy was three miles away and not dressed.
The police had been called by Neville ten minutes earlier.
Neville would be putting his only son in danger.
Jeremy may be scared to go over. He told Liz Rimmington that Sheila was going 'back to the nut house' and told the police she had committed child abuse. Now Neville had told him she had 'gone crazy' with a gun.
Jeremy did not have a key to WHF so would have to smash the door down, or smash a window. Or shout through the letter box. Making Sheila more excitable. Neville may not have been aware that Jeremy could get in through windows.
Sheila would not be pleased to see him.
Jeremy would be reluctant to rush over if answering the phone. Having poor relations with all the family.
Jeremy may refuse to go over. Being tired and upset at being woken.Lookout said it was common for Sheila to have 'one of her turns' so he may have decided to go back to bed.
Jeremy may not rush over. Maybe wasting up to 26 minutes doing strange things.
Jeremy may just ring nearby farm workers and tell them to go over. Something Neville could do himself and may consider a better option.
Jeremy may just ring the Foakes's and tell them to go over. Something Neville could do and may consider a better option.
Neville calling someone would antagonise Sheila. She may even brutally beat someone.
Neville could call the Foakes's. Who were two adults living at WHF.
There was no time to make any calls if Sheila had 'gone' crazy with a gun.
When there was time to make a call Neville had the option of restraining Sheila. Getting June to assist.
There was no time to wait for someone to arrive. Sheila had 'gone 'crazy'.
Relations between the two were poor and getting worse. After Jeremy robbed the caravan site and spent the money.
Neville would be prepared to take a torso bullet in order to restrain Sheila. Knowing the gun was for shooting vermin/rabbits and a torso shot would not kill him.
The kitchen phone was downstairs. Meaning everyone would be left upstairs. Unless Sheila had accompanied him downstairs.
If the answering machine was switched off, as his supporters claim. Jeremy would not hear his downstairs phone. If his bedroom door was shut.
Neville was bigger and stronger than Jeremy and the head of the family.
Sheila would have to let Neville open the door when Jeremy arrives. Or Neville would have to put himself in danger by getting to the front door against Sheila's wishes.
Sheila is more likely to fire bullets if more people start pressurising her and arriving on the scene.
Neville would be aware that he was the person who always had most success in calming down an excited Sheila. If he couldn't, then someone who thought Sheila was a 'looney' certainly could not.
Jeremy was not known to be especially brave. So may refuse to go anywhere near the inside of WHF. Preferring to stay 50 yards from the entrance. This is what happened.
Neville would be implicating his son if there were any injuries or fatalities. As Jeremy would be first on the scene and first witness. This is what happened.
Jeremy was not known for being especially brave. So may have just phoned the police. Neville had either already done this. Or wanted to keep things private. Jeremy did in fact call the police.
It is not standard practice to call relations on the phone at 3am when you're life is in danger. There are no other examples of this happening.