Omnipay\Common\CreditCard::getBillingPhoneExtension PHP Method

    public function getBillingPhoneExtension()
    {
        return $this->getParameter('billingPhoneExtension');
    }

 public function testPhoneExtension()
 {
     $this->card->setPhoneExtension('001');
     $this->assertEquals('001', $this->card->getPhoneExtension());
     $this->assertEquals('001', $this->card->getBillingPhoneExtension());
     $this->assertEquals('001', $this->card->getShippingPhoneExtension());
 }