When using a trial account with Twilio, all your account usage costs are free. This includes the one phone number you own as well as any voice calls and SMS messages you make and receive during your Trial period.
You may notice the billing information on your account's Usage page. This is because we want to give you a sense of how much it would cost if you weren't using a trial account. By displaying the amount on your Account Usage page, we hope to give you a better understanding of our pricing and your own account usage.
Since you haven't added any funds to your account, you are not charged. In addition, you will not owe this money once you upgrade your account.
Once you upgrade your account, then you will be putting money into your Account Balance. With an upgraded account, any Twilio usage through your account will be deducted from your account balance.