I unpinned my phone tile from my start screen on my Nokia Lumia 920, and now I don't know how to put it back. I can no longer view my call history or dial new numbers, how can I pin it back?
Very simple:
In the App list, tap and hold the app you want to pin, then tap Pin to start.
Source: Pinning things to Start
Swipe left to all apps, scroll to phone, tap and hold until you can tap pin to start.