How to Record a Voicemail Greeting for Your iPhone
You can record your own voicemail greeting for your iPhone. Your phone number comes with a default, generic recorded greeting (by now, we've all heard that pleasant-but-robotic voice telling us that, "the person at this number is unavailable" or something similar). There's no reason you can't keep the default greeting, or you can as most people choose: Create a more personal greeting in your own voice. The following steps describe creating a custom greeting for your iPhone.
Inside the voicemail application, tap the Greeting button.
Tap Custom.
Tap Record and start dictating a voicemail greeting.
When you have finished recording, tap Stop.
Review the greeting by pressing Play.
If the greeting is worthy, tap Save. If not, tap Cancel and start over at step 1.

iPhone 4S Glossary
FaceTime
Apple software that enables users with supported mobile devices to both see and hear each other during wireless conversations.

iPhone 4S Glossary
iCal
Apple’s personal calendar application.

iPhone 4S Glossary
iCloud
Apple’s wireless storage and synchronization service that keeps digital information updated automatically and wirelessly without user instigation.

iPhone 4S Glossary
iOS 5
The operating system for mobile Apple devices released in June, 2011. With iOS 5, data updates and synchronization occur wirelessly through the iCloud.

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jailbreak
Altering an electronic device to remove default restrictions imposed by the manufacturer.

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Siri
A voice-controlled personal assistant application purchased by Apple and available exclusively on the iPhone 4S.