The prompt will stop appearing. If the 611 Automatic System does not work for your carrier, you can head to your carrier‚Äôs official website if youriPhone 6s keeps asking for voicemail password. From the myWireless Account home page, select Phone/Device then choose the Reset Voice Mail PIN link from the left column. Note: If there is more than one line on the account, make sure the correct wireless number is displayed in the Wireless Number list before selecting Reset Voice Mail PIN. To reset the password for any of the lines on the account, choose a wireless number from the dropdown menu and click Submit. In a while, iPhone will display "Password Incorrect - Enter Voicemail Password." Enter the last seven digits of the wireless phone number and tap OK. Log into My Verizon with your cell phone number and password. If you've set up a username, use the username instead of your phone number. Select My Verizon> Reset Voice Mail Password. Choose your cell phone number from the drop-down menu, and thenclick on "I Will Create My Own Password." Enter and confirm your new password. Click on "Submit" to save the change. Sign in to My Sprint with your username and password. Click“My preferences” tab. Under Things I can manage online, click on Change voicemail passcode. Enter your new passcode and confirm. Tap Save and you will get a message informing you the new password has been saved.
By default, iMazing will export all the voicemail from the selected contact(s). If you only want to export specific voicemail, select them now. In the bottom toolbar, click Export. This will open a save panel dialog box. In the dialog box, select where you want to export your files, then click Choose. Use the CTRL/Command or SHIFT keys to select multiple contacts or voicemail. Double-click your voicemail to preview it. Transcript column

To avoid missing important calls in those situations, it is better if you know how to set up voicemail on Android or iPhone so that you can be away from other businesses or engagements without having to worry about missing any important calls.
I have also seen this method fix voicemails that are not playing and not deleting, too. A quick reset of the voice app does wonders and takes two seconds.