You can preformat the cell as Text (format|cells|number tab), then do your data
entry.

Or you can prefix your entry with an apostrophe:  '2-28-6

If you'll admit that it is not actually a number
and will not be used arithmetically, you can format
the column as text,  or precede the entry with a single
quote to make it text by overriding the format temporarily.

Otherwise Excel will treat it as a number and it has nothing
to do with how you format a cell as that is only to tell Excel
that it is a number and describe the format of the display.