Leading Zeros being dropped

I am importing a file into Power BI that contains tracking numbers for a large number of shipments. Some of the tracking numbers  are all numbers and some contain letters.  When I import the file into BI it automatically converts the tracking number column to a numbers field. When I change it to text it won't change the field back to it's original state with the inclusion of the leading zeros.  How can I prevent the leading zeros from being dropped?  Thanks.