Lawrence Phillips complained that prison staffers were screwing with him in the month before he died ... messing with his mail and breaking his TV ... according to a letter penned by the ex-NFL star.
TMZ Sports obtained the letter Phillips sent to his old high school football coach on Dec. 6th -- in which he implied staffers at Kern Valley State Prison were NOT delivering his mail.
"The mail situation is terrible," Phillips wrote.
"Everyone is pointing the finger at everyone else. I surely am not receiving all my mail as well as mail I am sending is not being received.