Yes, there is a relatively simple way to change the phone number format using a formula in Microsoft Excel. Here are the steps:

1. Assuming that the phone numbers are in column A, insert a new column next to it (column B) where the new formatted phone numbers will be displayed.
2. In cell B1, enter the following formula:
   Formula:

   =LEFT(A1,3)&"-"&MID(A1,4,3)&"-"&RIGHT(A1,4) 

3. Press Enter. The formula will split the phone number into three parts: the first three digits (the area code), the next three digits (the prefix), and the last four digits (the line number). It will then join these parts together with hyphens in the desired format.
4. Copy the formula down to the rest of the cells in column B to apply it to all the phone numbers in the list.

If some of the phone numbers are already in the desired format, the formula will still work. It will simply split the phone number into three parts and join them back together with hyphens, resulting in the same format.