SB according to herself was in close proximity to JM while she was on the phone to JB at the 10pm call .looking at thev "horror on Julie's face " after the 10pm call.
Surprised she didn't pick up and hear the dialog, if you stand in close proximity to someone on the phone you can certainly hear and here is an individual on the other end apparently planning to massacre his family in a few hours, he must have been angry and pumped.
I don’t think there was any planning during this conversation, the conversation was about days events. The planning had been done weeks before, Bamber was supposed to have said, “I’ve been thinking on the tractor and the crime had to be tonight” it had been discussed many times between them what he intended to do, so Julie was aware what he meant, “tonight is the night or never” so not a lot of dialogue or planning to massacre in that phone call in my opinion. They were both in on it, Bamber trusted Julie with the caravan robbery and his drug dealings, Julie trusted Bamber with her cheque book fraud and her part in the drug dealings, this was the next step for them both.