No that wouldn't be an issue. I can testify that what I am saying is true, because exactly the same thing happened to me in the 70's. If someone at WHF depressed the button for just a couple of seconds it would have the effect of the phone going dead. I fact the length of time the button is depressed is immaterial. But the leaving off of the receiver from the cradle is. For it is this that enables Jeremy to phone back and get the engaged tone. Now if Jeremy had phoned the operator the operator would have been able to tell him whether the phone was off the hook, or if Ralph was on another call. But he didn't. The important part in all this is that because the button was depressed for a couple of seconds making the phone go dead, Jeremy by putting his phone down would cause the engineer to believe that the call was ended by Jeremy and not Ralph.
This is contrary to the court testimony but if true you still have to find a reasonable explanation for why Nevill would call Jeremy. You ignored the second part of the inquiry.
Either Nevill called with Sheila watching him dial and speak or she was somewhere else as he called. So either he got away from her somehow and was away long enough for him to:
1) reach the phone
2) dial Jeremy
3) Jeremy to wake up, go downstairs and answer the phone
4) to pass his message to Jeremy
all before she either ordered him to push the button down or pushed the button down herself.
This would take several minutes to happen. In those several minutes he coudl have gone into the back kitchen, got a gun and loaded it, gotten a knife, or grabbed something else to use as a weapon and then gone to look for Sheila to disarm her.
Nevill had no idea that Jeremy did not have his answering machine turned off so had no idea Jeremy would answer. He had no idea Jeremy would wake up and answer evne with the machine off. If he was lucky enough to get Jeremy to answer the entire process would take at least 15 minutes for Jeremy to get there then he would need to find a way in.
Why would Neville decide not to make the effort to disarm her himself immediately but instead to run to a phone to try to get Jeremy to come do it 15-20 minutes later?
Worse yet, why would he not arm himself and go disarm her instead of grabbing the phone and being a nice easy target when she finds him and such call could even tick her off enough to make her shoot onsite?
It makes no sense to ask Jeremy to come disarm her instead of trying himself but evne less to not grab a gun or other weapon if he was left alone downstairs. Surely if he had enough time to dial, wait for Jeremy to answer and to even speak his message before she found him then he had time to grab a weapon.
If he didn't get away from her then why would she let him call Jeremy? Why would she just sit there watching as Nevill dialed, waited for Jeremy to answer, and even waited for him to speak to Jeremy before finally stepping in to end the call?
Neither scenario makes and sense and it would have to be one or the other.
Since it doesn't make sense the scenario put forth by the phone company and prosecution makes more sense and it the more believable account.