Either:

Highlight the column in Excel and select the General format.

or

Assuming the telephone numbers are in column F, insert a new column G with a formula like =Concatenate("(",left(F4,3),") ",Mid(F4,4,3),"-",right(F4,4))

Then either hide Column G or open column H, copy column G and "Paste Special"/Values into column H. Then delete columns F and G.