Voicemail notifications
I've studied the support page for this issue from other users, and it's consistently blamed on the carrier. Nevertheless, I thought I'd note my experience.
As soon as I switched to my new iPhone13, I noticed notifications of received voicemails are not showing up for anywhere from 15 minutes to 3 hours after they are recorded. If a call comes in that I can't or won't answer, and there's no subsequent voicemail notification, I assume the call is spam or unimportant. But today I missed an important message due to the notification not showing up. When the caller later pointed out that she left me the voicemail, I opened the phone app on my device and only then did the notification pop up.
I find it hard to believe this is my carrier's problem, given the behavior of the phone and the fact this never happened on previous iPhone versions. I've always had the same carrier.
iPhone 6, iOS 12