All zoom phone settings/preferences can be done in a web portal. In addition you can use the web portal to view your voicemail, call history, and even make calls.
Only offices have access to the Zoom portal. Classroom phones/users do not have this access.
To change the pin/voicemail for classroom phones please submit a helpdesk ticket.
You can also get to this if you are signed in at zoom.com and click Web App at the top → Phone:
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Click Phone at the top if not already selected:
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Use the left side to navigate:
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While in Voicemail, if you have multiple voicemail boxes, such as your phone AND a group line that has a voicemail, you can select which voicemail box to look at by clicking the '# Inboxes' dropdown:
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Click on a voicemail to listen to it, delete it, or delete all voicemail: