The Case Of The Burnt Letter: Part 1

An old man living alone in his apartment is choked to death by an unknown assailant. The motive behind the case is unclear, who would have any sort of enmity against a man of his age. His neighbor and friend noticed that he was tensed before his death, and seemed to have been calling someone moments before his death. Who was he calling? Why was he behaving so odd? ACP and his team are on it.