If you forget your voicemail password, you will not likely to reset it from your iPhone. You need to contact your carrier to reset your password.
In the Phone app , Visual Voicemail (available from select carriers) shows a list of your messages. You can choose which ones to play and delete without listening to all of them. A badge on the Voicemail icon indicates the number of unheard messages.

Rehearse or write down your message before recording it. Remember that old saying “practice makes perfect?” It’s certainly true when it comes to creating an electronic greeting. The more you’ve rehearsed, the easier the message will be to restate. If you don’t have time to practice, writing down the greeting before recording it – and then reading it aloud from the paper – may help you stay focused on the correct wording.
When you get a voicemail, you can check your message from the notification on your phone. Swipe down from the top of the screen. Tap Voicemail ….You can call your voicemail service to check your messages. Open your device’s Phone app . At the bottom, tap Dialpad . Touch and hold 1. How do you listen to your voicemail from a different phone? Call your 10-digit wireless number. When you hear your voicemail greeting, press the * key to interrupt it. If you reach the main voicemail system greeting, enter your 10-digit wireless phone number, then interrupt your greeting by pressing the * key. Can I listen to my voicemail without setting it up?