A man was walking towards the town centre along Stamford Road, close to the junction with Wellington Street, when he was punched from behind which caused him to drop his phone.
One of two men took it and ran off towards Wellington Street.
The robbery took place at about 8.30pm on Tuesday (April 4)
Both offenders are described as 25 to 30-years-old, 5ft 10in white men.
One wore a black Puffa style jacket, cream tracksuit bottoms with black stripes down the side and a cream baseball cap.
The other wore a cream jacket, black baseball cap and jeans.
Witnesses can contact police on 101 or Crimestoppers anonymously on 0800 555111.