Ajax callback never called / triggered
You're never making it to your callback because you don't have your #ajax array defined correctly, callback should not be defined as a hashed key. Take a look at the Adding an AJAX event example on d.o.
This should work:
$form['content']['validate_client'] = [
  '#type' =&gt; 'button',
  '#value' =&gt; $this-&gt;t('Validate Client'),
  '#ajax' =&gt; [
    // Change the callback key and update your callback value.
    'callback' =&gt; [$this, 'clientValidateForm'],
    'event' =&gt; 'click',
    'wrapper' =&gt; 'validate-phone-number',
  ],
];

Also note that Jaypan is right, you can't define new elements in your validation callback so you'll have to change that too. You may want to set a $form_state value in your validation handler and use that to add your element inside your build method. You may also need to update the wrapper value in your #ajax definition to align with the other changes.