A carrier account of ours. You connect your own — Twilio, Telnyx, Plivo, Vonage, SignalWire, DIDWW, DIDLogic, Wavix, a SIP trunk or your Asterisk — and the credentials stay yours, encrypted, on your account alone.
On a trial the carrier connection is set up with you rather than by you, because signing up is not a reason to be trusted with credentials that place charged calls on somebody else's bill.