When my niece moved in with us last year (long story), I moved her phone over to our account. I already had unlimited text messaging set up for the account, but apparently at some point while adding the extra phone line, the unlimited messaging did not carry over. I just about fell flat on the floor when I opened the bill at the end of the month and saw a bill for nearly $1400.
My niece had blown through more than $1200 in text messages in a month. I had received absolutely 0 notifications from the carrier, and when I called them, the support person agreed that the extra line should have had unlimited messaging, but they swore up and down that there was absolutely nothing they could do about the bill.
I got a manager on the phone, and he gave me the exact same spiel. At that point, I was ready to cancel the account. I wrote a very concise email to their customer retention department, and after two weeks, I got a response that my account would be credited for the overages. Sure enough, on my next bill, my account showed that the amount had been credited. But the next month after that, the credited amount was added back into the bill. Another email, and it was rescinded. This went on for about 2 more months, and I finally got in touch with a senior account representative in retention who finally explained what was happening.
Retentions was crediting the account, but Billing was still registering the overage as unpaid, and reapplying it with each new billing cycle. The account representative explained that her boss, an Executive VP with the carrier, ended up calling the billing folks and promised to start creating new orifices all around if they didn't get this situation fixed. Since then, no new overage fees.