Make sure that you have the latest version of iOS. Check for a carrier-bundle update: Go to Settings > General > About. If an update is available, a message will ask you to update. To check your cellular network connection, try to call someone.
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

The default voicemail greeting on the iPhone plays generic Your call has been forward to an automated voice message system recording. If you use your phone for work, create a personalized greeting that tells people you’re a professional.
Checking voicemail on Android from another phone. If you need to access your voicemail because your phone is out of charge or you are unable to access your phone, for the time being, you may do so from another phone as well: Call you own number from another phone; Let the recorded greeting play before it asks you to record your message.