Demonstrates how to add fields to a Participants Database Contact Form

pdb-contact-form-additions.php

<?php
/**
 * Plugin Name: PDB Contact Form Additions
 * Description: adds additional fields to the Participants Database contact form
 * Version: 2.1
 */
add_action( 'pdbcff-before_message_area', 'pdbcfa_add_fields' );

/**
 * inserts additional fields into the contact form
 * 
 */
function pdbcfa_add_fields()
{
  ?>
  <label for="pdbcff-phone">Phone Number</label>
  <input type="tel" pattern="^\d{3}-\d{3}-\d{4}$" title="phone number format: 123-456-7890" name="pdbcff-phone" value="" required id="pdbcff-phone" />
  <?php
}


/**
 * this demonstrates the use of the pdbcff-form_item_definitions filter to change how the contact form is configured
 * 
 * @param array $form_items
 * @return array form items array
 */
function pdbcfa_modify_form_fields( $form_items )
{
  // we are making the message body field not required
  foreach( $form_items as $key => &$item )
  {
    if ( $item['name'] === 'body' )
    {
      $item['required'] = false;
      // if you want to delete the item from the form use:
      // unset( $form_items[$key] );
    }
  }
  
  return $form_items;
}

add_filter( 'pdbcff-form_item_definitions', 'pdbcfa_modify_form_fields' );

Additional information on using regex patterns to validate the input:

http://html5pattern.com/Phones

Good place to test and debug your regex patterns:

https://regex101.com/

Edited to demonstrate use of the pdbcff-form_item_definitions filter to modify the internal fields.