from reddit:
After everything, Walter made that last phone call for one reason and one reason alone: to clear Skylar of all possible charges. Notice the language he uses- "you were always begging me to stop, telling me its illegal, immoral, someone could get hurt" etc, "I warned you what would happen if you crossed me", "Oh what am I saying, you never had anything to do with it, this was me, it was all me". Of course he presents it in a manner that is made to seem him like a piece of utter ****, and in many ways he is, but this was a truly selfless act to defend his family. Walt destroying the phone demonstrates he knew police were there, and solidifies my views.