Because she was in with it and he wanted to pre warn her in case the police questioned her before he had time to, he didn’t know how it would pan out once he had involved the Police.
What did JB want to warn Julie about that couldn't have been discussed/planned in the ten o'clock call, HB?
This was a long call I believe, something like twenty minutes.
If JB had really decided to kill his family that night, then that was the time to tell Julie that he was deadly serious and get their stories straight surely!
No ambiguous rubbish like tonights the night and nothing else.
And if Julie is telling the truth, nothing of any importance was said in the 3.15-30 call anyway!