I had the same issue with the phone. W left and then started using thousands of minutes. I wasn't concerned with the cost as much as that most of them were with a guy she used to work with. Anyway, my W said she'd pay for what she used so I didn't have that issue of asking her to pay. I haven't confronted her about who she's talking to and don't plan to...
As for you, it seems to me that it is perfectly fair to at least bring it up. Maybe she doesn't realize, maybe she could be more careful with the use and she just doesn't know she went overboard, who knows? Maybe you could be fine paying for it this time but let her know that you don't plan to do that in the future? Has money been a difficult subject or something you usually deal with fairly and rationally? The answer to that could point you in the right direction.
And I agree, the volunteering seems weird, but seems like a positive step.
Me: 43 W: 41 Together 2009, Married 2011 Sons 10 and 6, Daughter 5 Bomb 2/21/21. W moved out 10/2021