if ($this->journal3->settings->get('quickCheckoutAccountTelephoneField') === 'required') {
            $telephone = Arr::get($this->request->post, 'order_data.telephone');
    
        if ((utf8_strlen($telephone) == 10 ) || preg_match('/[^\d]/is', $telephone)) {
            $error['telephone'] = $this->language->get('error_telephone');
            }
        }

in opencart 2,3,0,2 Journal theme has another register.php, this works for validation for telephone.