Janice,
I had the same issue with my daughters but not $400, thank God.
Call the phone company and sign up for an unlimited text message package. Then you can ask for it to be retroactive. Sometimes they can do that -- maybe for part of the month, anyway. It won't solve the whole problem, but it might reduce your bill a little and definitely take care of any texting charges she's incurred which will appear on your next phone bill. BTW, adding unlimited texting wasn't that much more.
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No new car for me, my Honda CR-V's just fine for now.
The person below me, like me, is scared to death of mice.