If the phone downstairs was taken off the hook, it disables all phones.
Why would Sheila be downstairs if she wanted to massacre them ? And why didn't Neville & June get ready for her to come upstairs & then converge on her ? Neville was defenceless in his PJ's. June in bed.
The phone downstairs disabled all others as soon as Sheila took it off its cradle. Indications are,with having just one line,that use of another phone in the house,causes others to make a "ding ",which Sheila would have heard.
Sheila would initially have gone downstairs knowing that was where the guns/rifles were.
It wouldn't matter if you were in your bare skin,,you'd still be defenceless against someone brandishing a gun.
June wouldn't have been sleeping either,as she was a bad sleeper at the best of times and used to read her bible in bed.