We use the Contact Form 7 plugin in Phoenix, as it's simple & easy to customise to your needs.
Full contact form 7 details can be found here:
Contact Form 7We recommend to check this basic tutorial on how to use contact form 7 plugin
Setting up CF7Contact form 7 should be installed by default by the Phoenix plugins importer when you first install the theme, if not you can grab it from the "plugins" section of your WP Dashboard.
Contact form 7 lets you build your layout with shortcodes, see here for what we use in the Phoenix demo: