Creating tax exemptions for customers

Kentico 11 Website I have added a class CustomCustomerTaxClassService.cs for customer tax exempt in App_Code folder.

using CMS;
using CMS.Ecommerce;

[assembly: RegisterImplementation(typeof(ICustomerTaxClassService), typeof(CustomCustomerTaxClassService))]
public class CustomCustomerTaxClassService : ICustomerTaxClassService
{
    public CustomerTaxClass GetTaxClass(CustomerInfo customer)
    {
        if ((customer != null) && (customer.CustomerHasCompanyInfo))
        {
            return CustomerTaxClass.Exempt;
        }
        else
        {
            return CustomerTaxClass.Taxable;
        }
    }
}

and i am planning to change the code of GetTaxClass method as per my requirement but this method is never called when i am going through checkout flow.

At the same place, i have added CustomTaxCalculationService inherited by ITaxCalculationService to calculate tax with custom logic and it's working..

Is there something i am missing...