James Cook:

Here is how it works: When someone using iCloud Voicemail is unable to take a call, Siri will answer instead of letting the call go to a standard digital audio recorder.

iCloud Voicemail can relay information about where you are and why you can’t pick up the phone to certain people. But the coolest feature of the service is that Siri will transcribe any incoming voicemails, just as it does with anything else you say to it.

This makes sense to me. Google’s been doing this for quite a while now, but the service is surprisingly bad, considering how good Google voice is. Hopefully Apple will be able to figure out how to make the service work reliably.