That is one scenario.
I would automatically put the phone back on the hook.
How would Neville have got downstairs to make the call ? Assume June would be awake now & was being held hostage, but she put up no resistance before being shot in the head.
If Neville ran upstairs after hearing gun shots, he would have done so to save his wife, grand children as well as his daughter Sheila, even if it meant he died. Not to run away back down stairs straight away. The only other natural reaction after hearing upstairs gun shots would be to run outside & escape.