Tap “Phone” on your iPhone’s “Home” screen and tap “Voicemail” to access your voice-mail settings. Tap “Greeting” and select “Default” if you wish to use the default greeting for your voice mails. Click “Custom” if you want to create a personalized greeting, and tap “Record” to say your greeting.
When using a Cisco phone, you can record and change your voicemail greeting directly on the phone. For the following instructions, to select an option, you can use the phone's blue toggle button to scroll down and highlight the desired option, and then press the Select soft key. Press the Services (globe icon) button on the phone.

4. Reset your pin. Maybe you have forgotten your pin or passcode or you never set it up in the first place. This is a common issue. Most carriers have detailed instructions for changing passcodes. For example, to change a T-Mobile passcode, hold down the "1" key, then press the star key, and then press number 5 to go into passcode security. Then press 1, and you will be able to change your passcode. You can change your Sprint passcode online by choosing the my preferences tab and then the "things I can manage online" section. You can reset your pin by calling the carrier if you have forgotten it or never set it up at all. Some sites allow you to do this online.
If you have an Android phone, you might be able to disable voicemail by adjusting your call-forwarding settings. You can disable three functions, such as Forward When Busy, Forward When Unanswered, and Forward When Unreached. In some cases, you can disable all call forwarding. Check if you have call forwarding.