getQuote not working some stores

Question

I Need to restrict the tax amount. so i went Mage/Tax/Model/Calculation.php Then Finds calcTaxAmount() Tax Applying Function. I need to restrict tax who all are enter the tax vatid in checkout onepage tax should be zero so

public function calcTaxAmount($price, $taxRate, $priceIncludeTax = false, $round = true)
    {
        $billing = Mage::getModel('checkout/session')->getQuote()->getCustomerTaxvat();

        if($billing != "" )
        {
            return 0;
        }
        $taxRate = $taxRate / 100;

        if ($priceIncludeTax) {
            $amount = $price * (1 - 1 / (1 + $taxRate));
        } else {
            $amount = $price * $taxRate;
        }

        if ($round) {
            return $this->round($amount);
        }

        return $amount;
    }

I added the new condition. its working some stores of multistores. Only one store cannot working properly. it causes user cannot register , and addtocart not working for that particular store. i found getQuote the issue. i remove the new condtion like below working fine.

Old Function:-

public function calcTaxAmount($price, $taxRate, $priceIncludeTax = false, $round = true)
    {
        $taxRate = $taxRate / 100;

        if ($priceIncludeTax) {
            $amount = $price * (1 - 1 / (1 + $taxRate));
        } else {
            $amount = $price * $taxRate;
        }

        if ($round) {
            return $this->round($amount);
        }

        return $amount;
    }

Answer 1

I was Fixed Below Flow:-

Instead of Using GetQuote In Calculation.php Using Session Variable

Mage/Checkout/Model/Type/Onepage.php

Function Name:- public function saveBilling($data, $customerAddressId)

        $assign = $this->getQuote()->getCustomerTaxvat();
        if ($assign !="")
        {
            $this->_checkoutSession->setVatSession($assign);
        }
        else
        {
            $this->_checkoutSession->unsVatSession();
        }

Add Above code in the onepage.php before return array(); which means last of the function.

Now Below Get Accessing the Session Variable

Mage/Tax/Model/Calculation.php

public function calcTaxAmount($price, $taxRate, $priceIncludeTax = false, $round = true)
    {
        $sessionaccess = Mage::getModel('checkout/session')->getVatSession();
        if($sessionaccess != "")
        {
            return 0;
        }
        $taxRate = $taxRate / 100;

        if ($priceIncludeTax) {
            $amount = $price * (1 - 1 / (1 + $taxRate));
        } else {
            $amount = $price * $taxRate;
        }

        if ($round) {
            return $this->round($amount);
        }

        return $amount;
    }