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I'm not sure where you created the telephone value (was this using profile?). But to answer your specific question.
1) You need to add the 'user_viewtel' as a variable in the XoopsUser class (see ./kernel/user.php) starting at line 59. You'll want to add:
$this->initVar('user_viewtel', XOBJ_DTYPE_INT, 1, false);
You can add it anywhere between lines 59 - 90 but I'd probably put it just after the initialization for the 'user_viewemail' variable.
Then to retrieve the value from the database you'll use:
$thisUser->getVar('user_viewtel');
Similarly when you want to set the value in the XoopsUser class object to whatever is contained in $value, you would using something like:
$thisUser->setVar('user_viewtel', $value);
Doing this will probably require that you change forms, etc... too. This hack will also have to be re-applied if you ever upgrade your XOOPS installation. There's probably a better way to handle this than hacking core files.