General Questions/New to Joomla! 5.x • I try to add telephone field to the contact form

Code:

class PlgSystemAddTelephoneNumber extends CMSPlugin {    public function onContentPrepareForm(Form $form, $data)  { ... }    

Code:

<?php foreach ($this->form->getFieldsets() as $fieldset) : ?>            <?php if ($fieldset->name === 'captcha' && $this->captchaEnabled) : ?>                <?php continue; ?>            <?php endif; ?>            <?php $fields = $this->form->getFieldset($fieldset->name); ?>            <?php if ($fieldset->name === 'contact') {                echo 'Dit is de fieldset name: '."$fieldset->name: " . count($fields);// $fieldset->name: = contact and number of fields is 6                $telephoneFieldXml = '                    <field                        name="jform[contact_telephone]"                        type="text"                        label="Telephone"                        description="Please provide your phone number."                        required="false"                        filter="string"                        id="contact-telephone-field"                    />';                $telephoneField = new SimpleXMLElement($telephoneFieldXml);                if ($telephoneField) {// $telephoneField = true                    echo '<pre>Telephone field created successfully.</pre>';                } else {                    echo '<pre>Error: Unable to create the telephone field.</pre>';                }                $result = $this->form->setField($telephoneField, 'jform[contact_telephone]', 'contact');                echo '<pre>setField() result: ' . ($result ? 'true' : 'false') . '</pre>';// result is true                // Refresh fields after adding                $fields = $this->form->getFieldset($fieldset->name);                echo 'Dit is de fieldset name: '."$fieldset->name: " . count($fields);// Number of fields is still 6!!??            }            ?>                        <?php if (count($fields)) : ?>

Code:

<?php if ($field->name === 'jform[contact_email]') : ?> ... than adding here the field