duplicate address error

Hi,
I want to make sure the phone number each person writes is unique.
I have set the database cell to be unique, but I wan't to display error massage for the user.
can some one please tell me on how to do that? Nicolas, are you here?

Thanks for your help

ok, I did a little digging, and found out this:
in the file: /components/com_hikashop/controllers/address.php
on line 72 it's saves the address using this code:

$address_id = $addressClass->save($addressData);

if (!$address_id) {
				echo 'phone already assigned to user, please select a different one';
			}

if (mysql_errno() == 1062) {
    print 'phone already assigned to user, please select a different one';
}

Thanks for your reply.
but does this error checks to see if the error is duplicate phone or is it just to show an error if form could not submit it to database.
I have added another custom field which is password (would love to see password field in the custom fields options) and I wan't to check it for only digits and present and error for the user.
as I see in your solution that you made, the error massage is the same for all the errors when a user tries to save his address.
I wan't to check for the error number so I can present the right error massage to my users.

Thank you for your help.