Bamber said it was cut off - he was clear on that. What circumstances would have made him change from the situation allowing him time to phone Jeremy (who'd be asleep and could not be guaranteed to answer) - to being so urgent he cut off his own call mid-stream?
Nevill didn't cut the call off mid stream, he told JB all that he could at that stage ie, 'please come over, Sheila has gone crazy and has the gun'
Its a wonder Nevill didn't go and open the door to let him in, but I suppose he was anxious to get upstairs.
Or maybe he did unlock the door and Sheila bolted it again later! Pity no fingerprints were taken, such a loss of evidence.