Which iPhone application allows you to monitor your voice data (i.e. minutes in incoming/outgoing)?

You can find the voice data usage in  Settings -> Mobile data. Check voice data details in that screen. It will show you current and lifetime voice data in minutes. You can scroll down to the end of the screen and reset the current voice data usage. So, that you can keep track of your usage until you reset it again.

There is no such third party apps in Apple App Store. Apple doesn't let developers to create such apps on its platform for security reasons. In technical terms, Apple doesn't provide any API to developers to measure the voice data usage.