Teslas don't have to phone home to operate. They work just fine off-grid, and you can also opt-out of any data collection/phone home [1] if you want. As mentioned elsewhere in this thread, the burden of proof is on the manufacturer to prove that the modification caused the damage (at least in the USA), so removing the sim and turning off wifi is unlikely to void any warranty
[1] https://www.tesla.com/about/legal#choice-transparency