This happens when you have unwanted characters in the excel sheet copied from other sources.
The
way to make sure your data in excel sheet is clean is to copy the data
from source, paste it into notepad, and then copy from notepad and paste
it in to excel sheet. Notepad cleans all unwanted, hidden characters in
the text being imported.
Simply download our sample file
to use as the template and paste your notepad-cleaned data in that
sample file and then import it using the Import contact function.
PS.
This is not something you would be doing for long since we are soon
going to implement an automated cleaner to clean all the unwanted
characters from the source.