Excel automatically formats columns containing only numbers in to an integer or double.
This may cause some issues when trying to upload or match UK telephone numbers as they start with a leading zero and this is stripped by Excel's default formatting.
To add the leading 0, follow the below instructions.
1. Add a new column beside the existing telephone column.
2. Move to the first cell in the new column and enter the following formula in the cell: ="0"&[existing telephone cell reference]
3. Hit Enter.
4. Double Click the little square in the bottom right hand hand side of the cell.
5. Select the entire column and Right Click > Copy.
6. With the column contents still highlighted, go to the first cell in original telephone number column and in the Menu Bar, Edit > Paste Special… Paste as Text/Values.
7. Delete the formulated telephone column so you are left with the original column.
8. Save the document as a CSV.
Note that if you if you re-open the Excel file to check and hit Save - Excel will remove the leading zero and you'll have to go through the whole procedure again! The safer way to check is to open this CSV file in Notepad (Go to the CSV and Right Click > Open With > Notepad).