Phone app using too much battery? So let me preface my image by saying I have made ~20 seconds of phone calls in the past 24 hours.  

After heavily browsing the web in 1-2 bars of LTE coverage for a few hours, I looked at my battery usage statistics and saw the following.  What's the deal here? This is iOS 10.1.1 on an iPhone 7 on AT&amp;T.
You're just interpreting the figures incorrectly.

The percentages are not % of the battery used, but % of the usage by each app contributing any usage in the past day or week.  

Note it always adds up to 100%, no matter how much battery charge you have left, or when you last charged it.

It just happens to be, that of all the battery used since yesterday, your phone call used ⅓ of that usage - not ⅓ of a full charge.

Check the 7-day figure to see that also always adds up to 100.

It would be incredibly difficult to show the stats any other way.  

Imagine... you used an app on a full charge, recharged when it was barely used, used another app for 2 hours, recharged it for 20 mins, then the phone rang, you spent an hour on the phone, then recharged but only halfway, after which you used another app...
...nothing could keep track of that in any meaningful way.