In this tutorial, we will see how to avoid Error Code 21010 billing Tel required parameter missing . This problem occurs due to very minor mistake made by developer after connecting an application to the CCAvenue payment gateway. Due to some input values that are missing by developers while coding, such error occurs. CCAvenue is one of the leading online payment gateway service provider similar to other popular providers PayPal, Citrus, PayUMoney etc. Easy integration is an advantage of CCAvenue payment gateway. As per the guidelines of integration provided by CCAvenue that are necessarily needed to be followed while integrating it into the application. Input values like amount, currency, merchant_id, redirect_url, cancel_url etc are minimum required and should be included in the form before sending the data to CCAvenue page. If we avoid them or forgot to use them into the code, CCAvenue throws an error. As mentioned above, Billing tel is not the required input but if it is ad