I started using a 5s as a temporary replacement phone for a while a couple of years ago, and I couldn't register to iMessage, but I didn't dig really far (it wa

I started using a 5s as a temporary replacement phone for a while a couple of years ago, and I couldn't register to iMessage, but I didn't dig really far (it was a generic "registration failed" iirc). It might have been a fluke and not old phones can't register anymore.