If it's still under warranty, they HAVE to unless they can prove your modification caused the system to fail. It's no different than the silly stickers that say "warranty void if removed" - that's a nice fantasy for the manufacturer, but at least in the US it's been ruled those stickers mean absolutely nothing. If they're refusing to fix a phone under warranty, contact your local AG and enjoy watching them squirm. Loop in the FTC for good measure.

https://www.ftc.gov/business-guidance/resources/businesspersons-guide-federal-warranty-law#Magnuson-Moss