Start from the original value
Keep the source phone field unchanged and create a separate normalized field for checking.
- Preserves auditability
- Makes corrections reversible
- Allows results to be joined back to the source
Add the correct country calling code
A mixed-country list must carry an explicit country calling code for each row.
Do not apply one default country to every row unless the source is known to contain only that numbering plan.
Normalize common input problems
Remove display punctuation, blank rows, formulas, and duplicate normalized values.
- Store the column as text
- Check leading plus signs
- Review domestic trunk prefixes
- Use synthetic numbers in test files
Validate the export
Compare a small sample of source, normalized, and result values before using the exported file elsewhere.