To activate your new AT&T Voicemail Service by dialing your telephone number from the phone assigned with the feature, follow these steps: Dial your 10-digit telephone number and press * while the greeting message is playing. Create a passcode. Record your name. Record your personal greeting. Record your busy greeting. Enter *VM (*86). Tap the Call button. Follow the setup tutorial, which will walk you through selecting a password and recording both a voice signature and greeting for your voice mailbox. Note: If you hear your own or a system greeting, you’ve already set up your voicemail. From the Home screen, tap the Phone Key in the Quick Keys bar. (The Phone application can also be accessed from the Applications screen.) Dial *86 or press and hold to dial your voice mailbox. Follow the setup tutorial. Enter a password. Record a voice signature and greetings for your voice mailbox. Call your Straight Talk phone from a landline phone. When your voicemail begins, press “*” (star key) on your landline phone immediately. Enter the PIN you use to check your Voice Mail from your Straight Talk phone. Listen to the instructions.
Set up a professional voicemail greeting to ensure that those who need to contact you can leave a message to keep the discussion going. Another advantage of creating a professional voicemail greeting is that it reassures callers that they dialed the correct number, reducing the number of wasted messages and hang-ups.

While covering all of the relevant information, aim to keep your voicemail to about 20 seconds. You definitely should not ever record a business voicemail longer than 30 seconds, and anything less than 10 seconds will typically mean that you are either speaking too quickly or aren’t providing all of the required information.
If you've forgotten your retrieval number, call 800-244-1111 to get it. Alternately, the number was included in the welcome letter you received when you signed up for voicemail service.