Customizing a "contact us" button to include a required field for phone number

All I want to do is add a box so that they need to enter in there phone number. I cannot figure out how to do this. Currently, prospective clients click on the button, then a form pops up that has

Name:

Email:

Subject:

Message:

Please type the code you see on the left.

I want to add phone number to this. Please help.