If you've built an HTML-only form using CallRail's form builder, there is a specific technical limitation that prevents the use of the reCAPTCHA field we offer in our form builder. In Google's terms of service, they require any hostname for reCAPTCHA to be verified or whitelisted. For that reason, you're unable to use CallRail's reCAPTCHA field when you build a form that's HTML-only.
However, you can add your own reCAPTCHA to your HTML-only form once you've embedded your form on your website. We recommend installing reCAPTCHA v2 according to these instructions from Google:
You can also install Google's reCAPTCHA v3 to your form if you prefer. Here are Google's instructions for v3: