Why do the zeros vanish?

Excel reads 0123 as the number 123. Saved as text, they stay.

Only Japanese numbers missing their 0 (9 or 10 digits) are fixed; everything else is left as it is.

When postcodes and phone numbers lose their 0

012-3456 became 123456, 090... became 90...: Excel read them as numbers. Put the zeros back in one go.

Drop the CSV or Excel file; postcode and phone columns are found by name.
Postcodes are padded to seven digits, phone numbers get their 0 back, with hyphens if you like.
To open it in Excel, save as xlsx so the zeros stay, or turn on "Keep zeros when Excel opens it" for a CSV.
Check the count of fixed cells and save.

Why do the zeros vanish?: Excel reads 0123 as the number 123. Saved as text, they stay.
Numbers from abroad?: Only Japanese numbers missing their 0 (9 or 10 digits) are fixed; everything else is left as it is.

The CSV is never uploaded; everything runs in this browser.