It wouldn't support the telephone call, just that the killer was upstairs at the time. There can be many permutations of how and why Bamber might be upstairs and NB downstairs. My own thoughts (for what they are worth) is that Bamber didn't slink back in the night, he arrived back prior to Nevil going to bed - I think he forced an argument on purpose earlier and came back under the guise of an apology. He waited for Nevil to head off to the shower and went to work on the others - leaving only Nevil to deal with.
Yes it would support the telephone call in more ways than one.
The prosecution case is that NB was shot in his bedroom. Two of the gsw's NB sustained UPSTAIRS, lip and jaw, meant that purposeful speech thereafter was impossible so how could NB call JB "Sheilas gone crazy shes got the gun"? Furthermore these two gsw's produced heavy blood loss and no blood was found on the mouthpiece.
At trial Ed Lawson QC for the defence asked MF if any of the victims sustained any gsw's upstairs outside the main bedroom (obviously excl the twins). MF replied no. Ed Lawson did not ask MF how he arrived at this conclusion or challenge him in any way. I have not read Major Mead's (firearms expert for the the defence) testimony to know what he said or didn't say. However given Michael Turner QC at the 2002 appeal attempted to argue an officer used the kitchen phone to call out and inadvertently removed the blood it would seem unlikely Major Mead put up a robust defence at trial in this regard.
I've posted the link above re the construction at Bleak Farm. Anthony Scrivener QC comments that the totality of the physical evidence at soc by way of gunshot trajectories, casings and the gsw to the back of Fred Barras may well have caused Joe (ie average Joe/juror, I assume) to side with the prosecution as together it was strongly suggestive that Tony Martin did not fire all the shots whilst he was on the stairs as he claimed and fired whilst Fred Barras was attempting to escape.