Can't do it this way. The || will cast the string returned from param to a boolean. As other monks have pointed out do

if( (length(param('tele')) > 0) ||
    (length(param('mobile')) > 0) ) {

}
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However your logic doesn't match your error message - if you want to verify that the user has entered only one of the above you'll want to use && and if you want to make sure that they have entered at least one you'll want to warn when both tele and mobile are empty, i.e. length = 0 (although as pointed out before I would use defined to make sure the values aren't null).

You probably want to catch that just whitespace isn't being returned too...