The wording of this is perhaps a bit ambiguous. If you look at the documentation for the UIWebView property detectsPhoneNumbers in the XCode documentation browser, you'll see that it doesn't detect the user's phone number, but rather identifies phone numbers in the HTML.
Here's a snippet from the relevant documentation:
By default, a web view automatically
converts telephone numbers that appear
in web content to Phone links. When a
Phone link is tapped, the Phone
application launches and dials the
number. Set the detectsPhoneNumbers
property to NO to turn off this
default behavior.
The only way to uniquely identify an individual device is through its device ID property. This isn't an easy-to-read value, and only identifies the handset. So if a person buys a new phone, or sells their phone to someone else, the device ID is different.