By default, Apple does not allow ANY application to be the default dialer on an iPhone. That being said there are some great solutions.
#1 - Long Press the "PHONE" icon on an iPhone contact, and select "Productive Call Assistant". This will make Productive the default dialer for this contact moving forward. (See Image Below).
#2 - Simply go in to the Productive App, as you would your native iPhone calling app, and click on a contacts' name or review your call history in the "CALLS" tab, and tab on the contacts name. Productive gives you all the same functionality as your native iPhone dialer, AND MORE!