A2 represents where the broken number is stored.
=CONCATENATE("(",IF(LEN(A2)<9,"555",IF(LEFT(A2,1)="(",MID(A2,2,3),LEFT(A2,3))),") ",RIGHT(A2,8))
You should change 555 to your local area code.
Your presence here is welcome.
=CONCATENATE("(",IF(LEN(A2)<9,"555",IF(LEFT(A2,1)="(",MID(A2,2,3),LEFT(A2,3))),") ",RIGHT(A2,8))