There's an old saw that goes something like "The message intended is never the message received."

I can see someone who is signing up looking at your example telephone number, and saying to themselves, "Oh, that's a telephone number." and then entering theirs in a completely different format.

There are two ways to get the number in exactly the right format, most of the time. The irritating way is to beep at the customer when they enter it wrong and make them do it again. The better way is to give them three little fields and have the cursor jump automatically from one to the other. So they type 555 and the next number they enter will be the first digit in the second field. That works perfectly - as long as all of your potential customers have that number format, which means they're all in North America, I think.

Of course, there's no real requirement for any particular telephone number format within Zen Cart, but perhaps you want to export the phone numbers to another database?

Rob