Sure, and you could even reverse-engineer some messages, but you'd get something like
Phone must begin with "00" or "+" or nothing, followed by "1" or "7" or "20" or "27" or "3...
Whereas with a proper syntax description you could automatically insert a "+", delete a leading "00", and if the following two digits aren't a real country code have an informative error like
Please include the country code in the phone number
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Sure, and you could even reverse-engineer some messages, but you'd get something like
Whereas with a proper syntax description you could automatically insert a "+", delete a leading "00", and if the following two digits aren't a real country code have an informative error like