Julie mentioning that MM was going to phone Bamber, and use a memory phone, highlights that he planned to create the siege situation with the police. Which was the best idea.
People have claimed he should have blamed it on a random burglar of psychopath. Or let someone else find the dead bodies in the morning, with the gun on Sheila. While others have said he only telephoned the police on the spur of the moment to explain Neville being in the kitchen.
The memory phone was not available, but Bamber had to carry out the massacre anyway.