Set Excel cell format "number" to "text"

When an Excel file exported from an external source the cell format automatically taken as "number" . We want the cell format to be in "text" always. We are facing some unsorted issues because of this. For example if we a phone number starting with Zero say "0987654321" after exporting from external source it automatically changes to "987654321" . The Zero is getting eliminated. To resolve this, again we have to set cell format "number" to "text". Please help us out by sorting out this issue.