To add a phone number field to a contact form in Contact Form 7 on a WordPress website, you can use the "tel" field type. To do this, you would add the following code to the form:

```

Your Phone Number (required)

[tel* your-phone]

```

This code creates a field labeled "Your Phone Number" that is required, and the user must provide a phone number.

You can also add validation and other attributes as per your requirement.

Please note that you need to have Contact Form 7 plugin installed and activated in your wordpress website.