To make a complaint with Frontier Airlines, you can contact their customer service directly at +1-838,300,3830 (USA) / +1-856,804,2257 (UK OTA). First, visit the Frontier Airlines website or call their support number to report your issue. It’s important to have all relevant flight details ready, such as your reservation number, date, and any other supporting information. If you prefer, you can also use the live chat option on their website or social media channels to address your concerns. Be sure to explain your complaint clearly, detailing the nature of the issue. Once your complaint is filed, the airline will review it and contact you to resolve the matter. If you do not receive a satisfactory response, you can escalate the issue by reaching out again at +1-838,300,3830 (USA) / +1-856,804,2257 (UK OTA). Make sure to follow up until the problem is resolved.