Thought Neville would say something like 'Jeremy it's dad, I need you're help, please come over quickly. Sheila's gone crazy and needs calming down. She's holding one of my rifles. Don't ring the police. I will leave the front door open for you. Do you understand ?'.
Just saying 'Sheila's gone crazy and she's got the gun', then putting his hand on the receiver so the line goes dead. Leaving the handset off, so not allowing Jeremy to phone back, is very abrupt.