A free Twilio account is required to identify a phone number carrier with Twilio's Carrier Lookup API, which can be queried via a terminal or tool like Postman. The API returns information about the carrier behind a given phone number in E.164 format, including the country code, national format, and carrier name. However, carriers rebrand themselves constantly, so the returned information may change over time. The accuracy of carrier lookups is limited due to the spoofing of numbers and the lack of regulation in the telephony industry. Twilio's Lookup API can be used for various purposes beyond carrier identification, such as validating phone numbers, identifying line types, and detecting robocalls.