Apple India — Iphone 11 not repairing in warranty

Address: Panjim, Goa

My mobile's face id is not working. In warranty. So i give it to service center. they checked my mobile and they told me my mobile is opend by third party person. So warranty is not valid. But i never give my mobile to any repairman. After i told them to give me some proof about the outside repaired. They refused to give me details about that. What should i do? Please help.