The 19-year-old managed to keep hold of the phone before running away from her attacker, on the Silkin Way near Admaston, Telford.
She was walking at about 3.30pm on Sunday, February 3, when a man with a covered face punched her, causing minor injuries.
The man is described as white, in his late teens or early 20s, between 5ft 6ins and 5ft 7ins, and of slim build. He was wearing black jogging bottoms, a royal blue Puffa jacket with the hood up, black gloves and a black bag.
He ran off in the direction of Wrockwardine Road, Wellington.
West Mercia Police is appealing for witnesses, and anyone with information is asked to call 101 and quote incident number 518s of February 4.
Alternatively, contact Crimestoppers anonymously on 0800 555111 or at crimestoppers-uk.org