It's an unexpected improvement--really, who thought that the telephone could still be improved upon after all these years--but a welcome one.
FaceTime over cellular As long as we're on the subject of calling, there are a couple of other related improvements in iOS 6. FaceTime calls can now be conducted over the cellular network--however, there's a big asterisk on that: It depends on your carrier.
While Verizon and Sprint are placing no restrictions on FaceTime calling over cellular, AT&T has said that it will only allow users of its Shared Data plans to use the feature. It's a decision that has provoked a fair amount of ire from users who don't want to change their plans, but AT&T appears to be holding fast to that decision.
Given that I'm an AT&T customer, it makes trying out FaceTime over cellular a little tricky. However, thanks to a friend with a Verizon iPhone, I was at least able to see it in action. The verdict? It looks a lot like FaceTime over Wi-Fi. The only appreciable difference I could see was a bit more lag before my conversation partner on the other end responded--kind of like talking to the astronauts on the space station--which occasionally had us speaking over each other. But I can't say for sure whether that was a function of the cellular network or that particular connection.
If your data plan allows FaceTime over cellular, you can enable it by going to Settings -> FaceTime, and tapping the Use Cellular Data switch at the bottom.
Apple has also slightly improved the interface for which addresses you can be reached at for FaceTime. It now syncs between your devices, so you don't have to re-enter all your various email addresses and phone numbers. In theory, no matter which device you're using, FaceTime calls should be able to reach you on any of your emails or phone numbers. (Apple's done something similar with iMessage in iOS 6, for more on which see below.)
Do Not Disturb It's not specifically a phone-related feature, but iOS 6's new Do Not Disturb feature can help prevent you from getting bothered by unwanted phone calls while you're catching some shut-eye. The feature, which can be activated at the top level of Settings, lets you temporarily mute all notifications, including phone calls. When active, alerts will not sound and your screen won't even light up.
There are a few exceptions, which you can configure under Settings -> Notifications -> Do Not Disturb. For example, you can choose to allow calls from a specific group of your Contacts, or your Phone Favorites. ("Everyone" or "No One" are also options, if you're feeling more or less permissive.) If you're worried about not being reachable in case of an emergency, you can also enable a Repeated Calls feature--if someone calls you twice within three minutes, the second call will come through normally. Hopefully, the robocallers won't start programming their machines to take this into account, but I won't hold my breath.
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