UK area code
Rayleigh area code
Rayleigh is covered by 01268 and 01702. It sits across a boundary between dialling areas, so which one applies depends on which part of Rayleigh you mean.
01268The Basildon dialling areaSee what 01268 covers →01702The Southend-on-Sea dialling areaSee what 01702 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.