Excel removed the zero from my phone number — how to fix it
You typed or pasted in a number starting with 0, and Excel quietly turned it into a number without one. That's General formatting doing exactly what it's designed to do — treat anything that looks numeric as a number, and numbers don't have leading zeros. Drop your file in below to see which columns need to be forced to text first.
Why the zero disappears
A phone number like 0155512345 is a string of digits to you, but Excel's default General format looks at any all-digit entry and assumes it's meant to be a number. Numbers don't have leading zeros mathematically — 0155512345 and 155512345 are the same value — so Excel silently drops the zero the instant it decides the cell is numeric. There's no warning, because from Excel's point of view nothing went wrong; it did exactly what General format is supposed to do.
The three moments this happens
It happens on direct typing (type digits into a General-formatted cell, hit Enter, zero's gone), on paste (pasting a list of codes into an unformatted column re-triggers the same auto-detection), and — most often for larger files — on CSV import, where Excel's default "Open" behavior guesses each column's type from its contents and picks Number for a column that's actually meant to be text-like identifiers.
Why this isn't just phone numbers
The exact same mechanism strips leading zeros from ZIP codes (00501 becomes 501), account numbers, and product or employee codes that happen to start with zero. Any identifier that's digits-only but isn't meant to be used in arithmetic is at risk the moment it lands in a General-formatted cell.
How to prevent and fix it
Format the destination column as Text before typing or pasting values in, or prefix a manually typed entry with an apostrophe ('0155512345) to force text without changing the column format. For CSV imports specifically, use Data → Get Data → From Text/CSV (or the legacy Text Import Wizard) instead of just double-clicking the file, and set that column's data type to Text in the import dialog — that's the step that prevents the zero from ever being lost in the first place. Upload your file above to see which columns already lost their leading zeros so you know what needs re-entering from the source.