If I really have to list a phone number on a sign I prefer doing so without parenthesis. Dots or dashes will do fine. People understand it.
Most of the time phone numbers are totally useless items on signs. They consume valuable composition space on a sign face and are forgotten almost instantly. Unless you immediately write down the number or dial it into a cellphone chances are you will never remember it. How are you going to write down a number while driving? Many states are making cellphone use while driving illegal. Hopefully the business name will have made enough of an impact that you can remember that item and look it up in a phonebook.
Overall, phone numbers generally cheapen the look of a sign face. They make the business appear small time. Starbucks, Best Buy, McDonalds, IKEA and more never ever put their phone number up on their primary street and building sign displays. The extra information would clutter up the appearance and compete against the function of projecting brand identity.