Was it illegal for my cell phone company to ask for this information?

I’m doing battle with my cell phone carrier. I had a $242 credit on my account. Their new and “improved” billing system read it as a charge, and debited that amount AND the $250 for the real bill from my checking account.
I’m trying to get this fixed. They asked for a bank statement showing the debit. My bank faxed it over on their letter head. Then the cell phone company said they needed to see the balance before and the balance after the debit to “prove” that it actually debited from my account.
I had the bank send that, but the more I think about it, the balance in my account is none of their damn business.
In fact, the more I think about it, I think it may have been illegal for them to even ask to see it.
Anyone know anything about this?