UK area code
Dronfield area code
Dronfield is covered by 0114 and 01246. It sits across a boundary between dialling areas, so which one applies depends on which part of Dronfield you mean.
0114The Sheffield dialling areaSee what 0114 covers →01246The Chesterfield dialling areaSee what 01246 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.