Bruce Jenner -- Lesson Not Learned ... Talking on Cell Phone While Driving

Bruce Jenner isn't helping his cause any ... as authorities mull over whether to prosecute him for vehicular manslaughter, he was tooling around L.A. Friday driving around and talking on his phone.

Bruce finished a round of golf, jumped in his Porsche and took off. The later model Porsche's all have built-in bluetooth capabilities, but Bruce had phone to ear ... which is illegal.

Law enforcement has been telling us it's unlikely Jenner will be charged with vehicular manslaughter in the fatal car crash that occurred 3 weeks ago. One of the reasons he might skate -- he wasn't texting when he plowed into the Lexus.