UK area code
Wymondham area code
Wymondham is covered by 01603 and 01953. It sits across a boundary between dialling areas, so which one applies depends on which part of Wymondham you mean.
01603The Norwich dialling areaSee what 01603 covers →01953The Wymondham dialling areaSee what 01953 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.