A group of four teenage boys were playing in Cleveland Park when they were approached by two other teenagers.
The first robber approached one of the boys, aged 16, and asked for the time.
When the boy took his phone out, the other criminal knocked it out of his hands, picked it up off the floor and walked off.
He also picked up a mobile belonging to a 14-year-old boy which had been left on the floor.
The robbers then headed off down Cannock Road.
The incident was at around 8pm on June 5, police said today (June 11).
The first robber is Asian, 5ft 7ins tall, aged between 16 and 17 and of medium build.
He was wearing a navy blue polo shirt with pink lettering, dark grey jogging bottoms and a black and pink baseball cap.
The second criminal is 5ft 5ins tall, aged between 16 and 17 and of slim build.
He was wearing a light blue t-shirt and grey shorts.
Police are appealing for any witnesses or anyone who saw the offenders in the area at the time.
Anyone with any information should contact PC Lucy Hale on 101 or call Crimestoppers anonymously on 0800 555 111.