For security reasons, we have a "Usage Policy" set for every account. One of the things this policy does is generate a bill overnight for billable minutes if the monthly maximum is reached. If you have a SIP Trunking Lite account, it is possible that you will reach the maximum amount halfway through the month, which would then generate a bill.
Consider the following scenario: Your bill date is the 1st of every month. The basic usage policy level is at 50$, and by the 17th, you have reached 50$ of billable minutes. You will then see those billable minutes charged overnight on your credit card, even though it is not your monthly bill date.