Use $_POST after submit form
Your controller is in a new request. If you stored the data in the database retrieve it from there. If you did not store the data in submit in some place it is no longer available. You could use a session variable or a url query parameter to transfer the data from the form submit to the controller:

public function myFormSubmit($form, FormStateInterface $form_state) {
  ...
  $form_state-&gt;setRedirect('route_to_controller', [], ['query' =&gt; [
    'Tel_number' =&gt; '123456789',
  ]]);
}


and the controller

public function showNumber(Request $request) {

  $build['test'] = ['#markup' =&gt; $request-&gt;query-&gt;get('Tel_number')];

  return $build;
}