“When we visited with Tim Cook, he said that walking down streets in China one sees people speaking into their phones sending voice rather than text messages. Porting this capability to the watch makes sense as it is easier to send a voice message from a device already on the wrist than pulling out a phone. It also could aid penetration of China, which Cook said has a ways to go.”

Voice messaging on the iPhone also has a ways to go. If you want to dictate a message in iOS 7, you say your piece, wait for a server somewhere to translate it into text, and proofread the result to make sure you weren’t misunderstood.

Voice messaging in iOS 8 is more like voice mail, only instant. You don’t have to listen to your carrier explain how it works for the umpteenth time. You just touch a microphone button, record a message, and swipe the screen to send it.

It’s not hard to imagine doing that on a watch. Milunovich sees Chinese-style voice messaging as one of the chief  “jobs to be done” — along with encouraging wellness — for the iWatch Apple is rumored to be building in large quantities for release in October.

Assuming they get it right, Apple will probably sell millions of the things. The only question, says Milunovich, is how many millions.

“We model a unit ramp similar to the iPad at 21 million units in fiscal 2015 and 36 million units in fiscal 2016 at a $300 average selling price. Some analysts look for 40-50 million units the first year assuming 10-15% of iPhone users buy the iWatch. That’s certainly possible if Apple gets the product right but recognize that it’s a different kind of decision [than choosing to buy a phone]. The good news is that consumers interested in the iWatch may be less price sensitive.”

Below: Screenshots of how voice messaging works, via Apple’s iOS Preview.

Follow Philip Elmer-DeWitt on Twitter at @philiped. Read his Apple coverage at fortune.com/ped or subscribe via his RSS feed.