There is no in admin setting to manage this yet. In the future we will have ability to build custom forms.
What you can do now is to do following simple edits in the code:
1. Open file storefront/controller/pages/account/create.php
Locate line: 119
$this->data['form'][ 'telephone' ] = $form->getFieldHtml( array(
'type' => 'input',
'name' => 'telephone',
'value' => $this->request->post['telephone'],
'required' => true ));
Change : 'required' => true to 'required' => false
2. Open file /storefront/model/account/customer.php
Locate line : 87
if ((strlen(utf8_decode($data['telephone'])) < 3) || (strlen(utf8_decode($data['telephone'])) > 32)) {
$error['telephone'] = $this->language->get('error_telephone');
}
Comment or remove line:
//$error['telephone'] = $this->language->get('error_telephone');
Done