UK area code
Biggin Hill area code
Biggin Hill is covered by 01689 and 01959. It sits across a boundary between dialling areas, so which one applies depends on which part of Biggin Hill you mean.
01689The Orpington dialling areaSee what 01689 covers →01959The Westerham dialling areaSee what 01959 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.