I am glad he didn't pick the phone up.

At the start of the film he was looking into the abyss; and by the end he was a hero of sorts. So why spoil/complicate that by getting involved with a woman who tried to spike his major case?

I didn't agree with him slapping her mid-film, but I supported his decision to not pick up the phone at the end. Let her live with the moral consequences of her actions while staring at the bottom of whiskey glass!