Usability: phone numbers and special characters

[2010-09-24] dev, hci, webdev
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Why do so many web sites insist that, when entering a phone number, you not type any non-digits such as spaces, hyphens, parentheses or slashes? The structure that these characters bring to phone numbers help humans considerably and computers can easily filter them out. So what is the harm of allowing them? I would even store phone numbers in a database as they were entered.