UK area code
Chorley area code
Chorley is covered by 01257 and 01772. It sits across a boundary between dialling areas, so which one applies depends on which part of Chorley you mean.
01257The Coppull dialling areaSee what 01257 covers →01772The Preston dialling areaSee what 01772 covers →
If you have the number already, the code it starts with is the answer — that's more reliable than the town name for a place split like this. Look it up.