How to check field value against same field on existing nodes of same content type
I don't know how to achieve that with rules, but you could use EntityFieldQuery, in a custom submit callback of property_node_form.
Something like that :



/**
 * Implements hook_form_FORM_ID_alter().
 */
function mymodule_form_property_node_form_alter(&amp;$form, &amp;$form_state, $form_id) {
  $form['actions']['submit']['#submit'][] = 'mymodule_property_node_form_submit';
}

function mymodule_property_node_form_submit(&amp;$form, &amp;$form_state) {
  $phone_number = $form_state['values']['field_phone_number'];

  $query = new EntityFieldQuery();
  $query-&gt;entityCondition('entity_type', 'node')
    -&gt;entityCondition('bundle', 'property')
    -&gt;propertyCondition('nid', $form_state['node']-&gt;nid, '&lt;&gt;')
    -&gt;fieldCondition('field_phone_number', 'value', $phone_number)
    -&gt;range(0, 1);

  $same_phone_count = $query-&gt;count()-&gt;execute();

  if($same_phone_count &gt; 0) {
    // send mail
  }
}