Generally text fields in .CSVs are denoted with double quotes (") around them.
But you probably don't want to go through every row in Notepad and add quotes....
So in Excel, in a new column, use a formula to change the format. Let's say your phone numbers are in column A and you have a row of headers. So in some other empty column, type in =Text(A2,"000-00-0000") and drag/fill the formula down for all rows.
But you don't want to store this as a formula, so select that entire column and Copy. Select the column that contains the problem-phone numbers (without leading zeros) and Paste Special > Values. This will convert the formulas to text values. Now delete the formula column.
Also, you can eliminate this entire problem by correctly entering the phone number in the international format:
+<intlcode>-<area code>-<phone number>
like +91-22-12345678