http://www.macmillandictionary.com/dictionary/british/is
For me the word is clearly "if" and definitely neither "is", or shes, or anything else, i.e.:
"I wanted to ask Reeva why
if she's phoning the police"
I hear if just listening to the video without aids as well as after making a loop with a word either side and listening to it at normal, higher and lower speeds. I've also compared it with OP's "is" and it sounds nothing similar. They are completely different sounds because the f is a strongly aspirated sound (with a big puff) formed between teeth and lips while an "s" as an end letter (except when part of a double consonant) is pronounced iz as in fizz which is an unaspirated sound formed by the tongue vibrating (buzzing) behind the teeth up near the top palette.
But is it important? IMO no. Neither Nel nor the PT picked it up when hearing it in person (it would have been easier to distinguish for sure) and I am sure Nels acutely focussed and attentive ear would not have missed it. And it wouldn't prove anything anyway as it could be explained in so many ways, e.g. a simple stumble or mistake due to OP's nervousness and emotional state under cross, the strange SA syntax, or in its context with the part of the cross to which it pertains, it could it could only too easily argued OP was going to say he wanted to ask Reeva
... why wasn't she phoning the police, but changed his mind mid sentence to the less accusatory, "...if she was phoning the police, so Nel wouldnt accuse him of blaming her yet again. So too much reasonable doubt IMO to even bother as it is just another red herring as we all change track mid sentence x times a day (Nel included;-)) without any sinister connotation being made, so I dont expect it to even be mentioned in the PTs final submission nor reflected in Masipas judgement as a part of her findings either.
JMHO