When you have new voicemail, the Phone tab in the Skype for Business main window displays the number of your messages. See Contact Card opens the caller's contact card, which lists their phone number, email address, office location, and so on. Open Item in Outlook provides more information about the call.
We’re sorry to hear about this, if you can reach out to us on [email protected] with your details we can try to assist you further, however, we would need to confirm if you are able to manually access your voicemail through your phone. We can’t provide assurance or support for third party applications. However, if you encounter issues with manually accessing your voicemail through your phone we can assist with that 🙂

Website: https://support.8x8.com/express/devices/voip-phones-headsets/8x8-express-how-to-create-or-change-outgoing-voicemail-message
4. Setting Up a T-Mobile Voicemail: Open your iPhone's Phone app. It's the white phone icon on a green background on the Home screen. Tap "Keypad". It's one of the options at the bottom of the screen.