UK area code
Abingdon area code
Abingdon is covered by 01235 and 01865. It sits across a boundary between dialling areas, so which one applies depends on which part of Abingdon you mean.
01235The Abingdon dialling areaSee what 01235 covers →01865The Oxford dialling areaSee what 01865 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.