@frequent reader:
Last January, my iPhone 6 #1 started randomly restarting. It happened mostly when I wasn't using it. I was only aware that it had restarted because when I went to use my phone, I would see the message that touch ID was required after a restart. I would then find my wifi disabled and the button to turn it on was greyed out, so it couldn't be toggled back on. I was able to get it working again by various steps, ranging from just turning the phone off and back on, hard reset, or resetting network settings. The issue just got progressively worse over 3 months, where it would take progessively aggressive steps to get the wifi able to toggle on again. Eventually, it reached the point where the bluetooth also stopped working. A factory reset without restoring a backup showed no wifi or bluetooth function. At that point, Apple agreed it was a hardware issue and they replaced the phone.
Whole story is here:
https://forums.macrumors.com/threads/iphone-6-woes.1969877/
I wanted to mention that through all of this, I have not had Apple Care, yet I had phone support from two techs. I was within my warranty period for phone #1. The last tech I dealt with said that my problem with the second phone was Apple's fault, so it was replaced at no cost to me at all. Apparently my Canadian issued phone should not have been replaced with a U.S. issued phone in the first place. The one I got from the U.S. simply stopped working 30 days after I got it and I was back in Canada.
While I am pleased with the support I got, I am wondering if, had I been in this position with another brand of phone, I could have simply received a replacement from the vendor. (At the time the first phone had problems, however, I was not in a position to visit the original vendor to get a replacement.) It's rather a nuisance that one cannot just get a replacement authorization from Apple and go to one's original vendor with that for a replacement, rather than all the hassle of the item being shipped from Apple. (I had a rather harrowing experience getting my replacement phone due to poor communication from shipping companies, but it all worked out in the end.) This is something I will be considering when my contract is up with my carrier and I will be able to get another smart phone at a discount.