There have been some discussions revolving around the idea of potentially making calls free between e.g. family members on the same account, and how that might complicate billing or end up raising costs down the road compared to charging the usage for both phones as it is now. This suggestion is a compromise between those two: charge for one phone's usage.
The idea is that when one person makes a call to another phone on the same Ting account, the caller's phone gets charged the minutes, while the receiver's phone rides free. That way the usage is being paid for, yet there doesn't appear to be a "penalty" for talking between family members on the same account. (I realize the numbers aren't quite that simple on the back end, but I leave that for you to sort out!)
I pick that method of accounting because it maximizes the amount of usage for billing purposes without being overly complicated, like trying to charge both phones half minutes, or something equally odd. Another possibility might be to add a separate "device" named Shared for accounting purposes, if that works better for people, but I suspect "charge caller phone" is the easiest option to understand.
The same approach would likely make sense for text messages, but has no meaning for data megabytes.