I’ve tried a couple times now to switch my iPhone phone/data/text service from AT&T to Tello. I managed to get my daughter on their service with her iPhone 5 and later her iPhone 6. No problems there. My iPhone 6s didn’t want to go, despite AT&T claiming it was unlocked. Maybe the phone started life on Sprint and they hadn’t unlocked it. That’s possible.
Tonight, I switched my new-to-me jet black iPhone 7 256GB from AT&T, where I’ve used it a couple days, to Tello. No problems. Fast internet. So far (half an hour in), so good. I’m getting 200 voice minutes, unlimited texting (which I won’t use much of since I text few people without iPhones) and 1GB of data per month. $5/mo more to double that to 2GB/month. We’ll see how 1GB goes.