If this is CSV format, then this is not possible because customer numbers can have up to 6 characters and string fields are contained in quotes. Make sure that this is the case.

Do the import files get edited in Excel? This can cause problems with CSV-formatted files and also string fields that contain strictly numeric characters (which happens to be the case with these customer numbers).

Also with Excel, make sure that column widths are 'auto-fit' - I have seen strange things when a column is shrunk in Excel and all information in the cell is not able to be displayed. This can be avoided by selecting the whole sheet and choosing to auto-fit columns.