You can change the greeting that your callers will hear by clicking on the Greetings tab under Settings > Messaging.
There are two default greetings to choose from under the Configure Greeting section:
System with Number: Callers will hear the generic system greeting and your telephone number
System: Callers will hear the generic system greeting
If you record a personalized greeting and a name, you will see two more options appear:
System with Name: Callers will hear your name recorded in your voice with the generic system greeting
Personal: Callers will hear your personalized greeting