UK area code
Fareham area code
Fareham is covered by 023 and 01329. It sits across a boundary between dialling areas, so which one applies depends on which part of Fareham you mean.
023The Southampton dialling areaSee what 023 covers →01329The Fareham dialling areaSee what 01329 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.