UK area code
Tenterden area code
Tenterden is covered by 01233 and 01580. It sits across a boundary between dialling areas, so which one applies depends on which part of Tenterden you mean.
01233The Ashford (Kent) dialling areaSee what 01233 covers →01580The Cranbrook dialling areaSee what 01580 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.