When using a trial project with Twilio, all your 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 project'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 project. By displaying the amount on your Project Usage page, we hope to give you a better understanding of our pricing and your own usage.
Since you haven't added any funds to your project, you are not charged. In addition, you will not owe this money once you upgrade your project.
Once you upgrade your project, then you will be putting money into your Project Balance. With an upgraded project, any Twilio usage will be deducted from your project balance.