One credit = one minute of AI talk time. Your plan includes a bundle of minutes each month.
If you are running low, you get a warning text at 75% and again at 90% of your included minutes.
If you run out, nothing is charged automatically past your cap. You get a text asking if you want to top up, and nothing happens until you reply yes.
You can buy a credit pack any time from your billing page. Unused credits carry forward.