The problem could be a result of a virus or software bug on your phone. Before resetting your phone back to its factory settings, back up all of the information stored on it to another device, so that you can put it back onto your phone after the reset.
If the problem is with your phone, the factory reset has a good chance of solving it. If not, contact your mobile service provider, to carry out some troubleshooting to help resolve the problem from their end.

Tap the Phone app to launch it. Tap Voicemail in the lower-right corner of the screen.Find the voicemail you want to delete. Tap Delete (or, on some versions of the iOS, the trash can icon) to remove the voicemail.
It only take a few minutes to set up your Voicemail account. Once setup is complete, your Voicemail will be ready to receive messages. Call 123 to connect to your voicemail. If prompted for a password the first time you access your account, use the last four digits of your mobile number. When prompted, create a password which can be any 4 to 7 digit code. (A good password is one that’s easy for you to remember, but hard for others to guess) When prompted, record your greeting and name. Your voicemail is now ready to use. Call *86 to connect to your voicemail. If prompted for a password the first time you access your new account, use the last four digits of your mobile number. When prompted, create a password which can be any 4 to 7 digit code. (A good password is one that’s easy for you to remember, but hard for others to guess) When prompted, record your greeting and name. Your voicemail is now ready to use. Accessing your Voicemail Call 123 to connect to your voicemail Press 1 to listen to unread messages If you wish to replay a message, press 1 Press # to skip to the next message Press 7 to delete a message, or press 9 to save for the next 30 days At any time, press * twice to return to the main menu To change your password, press 5 from the main menu Call *86 from your phone Press 1 to play your messages Press # to skip to the next message Press 7 to delete a message, or press 9 to save To return to the main menu at any time, press * To enable or disable voicemail notifications, from the main menu press 4 for personal options, followed by 1 To change your password, press 4 for personal options, 1 for Administrative Options, then 1 again Checking Voicemail from a Landline Call 1-805-637-7249 Then when prompted, enter your 10-digit mobile number. You can skip the greeting by pressing the * key. When you’re prompted, enter your password. And finally, just follow the menu prompts to check your messages. Call your cell phone from the landline Let it ring till it goes to voicemail As soon as you hear the greeting, press # When you’re prompted, enter your password just follow the menu prompts to check your messages.