Hushed offer the ability for users to test out Hushed without making any purchases, they offer a free three-day North American trial number, so you can test out the features of the app. Once your three-day trial is over, the number will expire automatically, they do not charge or capture any payment information for the trial to be claimed, so they do not charge or automatically renew trial numbers.
All you have to do is let the number expire automatically, no need to cancel or delete the number before then, and it will simply expire from your account when the three days are up.