The incident happened at Dudley bus station on October 31 and left the young victim shaken but not hurt.
Now, after a review of CCTV footage, police have released images of a boy they believe is responsible for the robbery.
He is thought to be in his mid-teens and 5ft 8ins tall.
A spokesman for West Midlands Police said: "We need to speak to him about a robbery in Dudley bus station in which a young boy was threatened with a machete.
"The 13-year-old victim wasn’t hurt but was understandably shaken up by the incident on the evening of 31 October.
"We viewed CCTV and believe this person, who we suspect is in his mid-teens and is 5ft 8ins tall, is responsible for the phone robbery."
Anyone who recognises the boy is asked to contact police via Live Chat and quote 20/1853620/21 or call Crimestoppers on 0800 555 111.