Clue by Clue: The Bumped Off Banker Solution
The proof that someone killed Benjamin is that the phone was hung up after he died. Benjamin was on the phone when he died. But the assistant had to pick the phone up to call the police. That means someone hung up the phone after Benjamin died, but before the assistant came in. It must have been the killer; no one else had time. The killer probably didn't want the lawyer to hear any more details of the murder.