What do you mean the prosecution agree when the call happened? I thought they said there was no call. 
The prosecution agree the call could only have happened after Nevill had been shot 4 times. But say the call did not happen because -
a) His father was too badly injured to have spoken to anybody;
b) The telephone in the kitchen was not obviously blood stained;
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The defence did not counter by saying the call could have happened before Nevill had been shot 4 times. Bamber's evidence of what Nevill said makes this too ridiculous to suggest.
The OS have never disputed that Nevill made his call after the 4 shots.