I always settle my mobile phone bill with a credit card. For the last two months I've paid the bill as normal. The mobile phone company shows the payment received on the statement and a zero balance, but the credit card statements show no payment at all. Have paid for other things with the same credit card and they register no problem. So I'm effectively getting a free mobile. 
So would you
a) Raise it with the phone company
b) Raise it with the credit card company
c) Keep quiet and hope no-one notices!