Mailform Customization

Would be great if anybody could help me.

I need an additional line in my mailform with the telephone number of the customer, but i cannot find the place where to change this option. (Screenshot of the current mailform is included)

Thanks in advance - Juergen

What you see is the default Joomla component for contact/mail form.
It's not possible to change and/or add a new field.

You need to install a new component which will allow you to create custom forms with fields that you are requiring.

extensions.joomla.org/extensions/contact...edback/contact-forms