I found a way with 3 formulas:

For the Area Code:  Part (Phone Number, 2, 3)

For the Rest of Phone Number:  Part (Phone Number, 6, 8)

To Join Them:  Join (Area Code, "-", Rest of Phone Number)

You can change the joining formula to format it the way you want it.

You could probably nest this as Join(Part(Phone Number, 2, 3), "-", Part(Phone Number, 6, 8))

If you don't want the hyphen/dash between the area code and phone number, replace "-" with " " (a space between the quotes).

If you don't want the hyphen/dash between the area code and phone number, replace "-" with " " (a space between the quotes).