>>I have an application that has to run in 14 languages and 17 countries. I check for counrty, use this method for U.S. and allow 'free form' in non-U.S. because of all the weird(to me!) phone formats around the world.
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>I like this idea for U.S. phone numbers too. Is there any reason to format US phone numbers, especially when some have extensions and some do not?

Yes, there's an insurmountable one :) Users do not like it from a readability aspect. Extensions go into a separate field, as they vary in length.