The reason could be due to Excel processing mathematical symbols such as + (plus) and - (minus) as a mathematical formula, such as country codes for telephone numbers, e.g. +81, +44.
For example, +81-3-3333-3333 may be processed as -6588.
In order to fix this, change the data format of the Excel column to "character". Please refer to Microsoft's own help and support if you need more specific help with Excel.