You have to pay for API use, which is charged by the amount of data.
The expiration of a free trial credit is three months after account creation, not when you first discover and try to us the API.
You can either make a new account with new phone number and unique credentials, or simply pay for the minimum $5 credit.
There’s a chance you get offered a bonus discount as a few others have reported, so you get $10 for $5.