There could be a couple of possibilities in the call, we don’t know the tone of Nevill’s voice, maybe there had been an argument in the kitchen with him and Sheila without the gun, he calls Jeremy to give a hand as she’s being particularly aggressive (could the scratch marks come from here?) he doesn’t want to get June involved as he doesn’t want to risk her being hurt with Sheila in that mood so calls Jeremy whilst there is a stand off. The call is intended to be “can you please come and give me a hand? Your sisters being difficult” And the call starts in that vain, however whilst he’s talking Sheila grabs the gun for the first time and runs off - so his “she’s got a gun” is actually a separate thought/point and exclamation as he puts the phone down to chase her.
Or possibly another possibility is that she had the gun the whole time but he didn’t believe she would do anything with it, he calls Jeremy to help because he’s concerned that if he tries to take the gun off her then it could accidentally go off and injure either him or her, same for not getting June involved but he thinks Jeremy’s extra strength (or ability to negotiate) might work to avoid someone accidentally getting injured. Whilst he’s waiting for Jeremy to answer He and Sheila are still arguing across the kitchen which is why she doesn’t run off till he’s distracted by now speaking to Jeremy on the phone once he answers, when the phone is just ringing he can still focus on talking to Sheila.
Both of these are only suggestions - if the call took place then no one will ever know what really happened it can only be guessed at.