I'm sure he would have heard the phone, if there had been a call to hear.
That depends on how soundly someone is sleeping not just the ability to hear a phone from another room/level. Some people don't even wake up from thunder/explosions.
In the meantime how long it takes to wake up, decide to answer and actually get to the phone will ipact whether the caller gives up before you can reach it.
In so far as whether there had been a call to hear...
If you are truly panicked do you grab a shotgun/knife/some other weapon and try to confront her or do you instead waste several minutes on the phone knowing even if the person answers it will take at least 10 minutes for such person to dress and come over to try to help?
If you are too scare to disarm someone yourself and too scared to grab a gun to confront such person with you would call 999 not Jeremy.
The call makes no sense on so many levels and is contradicted by the shooting commencing in the bedroom while June and Nevill were in there. This is one of the things Jeremy supporters have a hard time overcoming. They are willing to ignore such but that doesn't make it go away that just is a way for them to rationalize what they choose to accept. Establishing innocence requires demonstrating to others that this call happened. It also requires demonstrating Sheila actually killed everyone including herself with the gun because if she didn't then it is not credible that Nevill would call claiming she had the gun.
So the first step of proving the call is possible even is establishing Sheila did kill everyone including herself.