A shop assistant working at Vodafone in Skegness chased an armed robber through a shopping centre before being threatened with a knife.
The offender is believed to have stolen an iPhone worth £1,000 from the store in the Hildreds Shopping Centre at around 1.30pm on Wednesday, January 17.
He was chased by the shop assistant to the Home Bargains exit point of the shopping centre when he was threatened.
Lincolnshire Police have said that the offender is described as white, late 20s, of medium height.
He spoke with a local accent and was wearing all black, including a puffa coat with a peaked hood.
Anyone with information should call 101, and make it for the attention of investigating officer DC Taff Lloyd.
Witnesses should quote incident 205 of January 17.