As mentioned in the previous post, you have to create a calculated column to add the formatting:

eg for a 10-digit number:

"(" & LEFT({Phone number},3) & ")" & MID({Phone number},4,3) & "-" & RIGHT({Phone number},4)

If you have a mix of 7 and 10-digit phone numbers, you have to create a condition that checks for the phone number field length and if 7 then format one way and if 10 format another way.