UK area code
Amesbury area code
Amesbury is covered by 01722 and 01980. It sits across a boundary between dialling areas, so which one applies depends on which part of Amesbury you mean.
01722The Salisbury dialling areaSee what 01722 covers →01980The Amesbury dialling areaSee what 01980 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.