Did you know that you can turn off Voicemail? Follow this link to find out how to turn off voicemail on iPhone.
After setting the number on your voicemail, you must also record a personal voicemail greeting. Follow the steps below. Start the Phone app. if you don’t see the numeric keypad, tap “Keypad” at the bottom of the screen. Tap and hold the “1” for about two seconds until the phone automatically calls voicemail. Immediately tap “3” to access voicemail settings. Listen to the options. On most voicemail systems, tap “2” to start the process to record a new greeting. The current personal greeting will play; then tap “2” again to record a new greeting. Say the greeting you want, and tap “#” when you’re done. To save the greeting, tap “1.” The personal voicemail greeting has been set. Hangup the phone. How to setup voicemail using a Visual Voicemail Applications

Visual Voicemail lets you see a list of who left you messages and then play or read voicemail in any order. YouMail lets you access your visual voicemail on your phone, tablet, or computer. You can choose what to listen to, which ones to delete or save, and easily call or text people back.
Sprint: Sprint has its own basic voicemail for you to set up. In order to get there, you’ll need to press and hold the 1 button and create a four-to-ten digit passcode to access your voicemail. You’ll then need to follow the voice prompts to set it up.