You can delete Voicemail messages by tapping on the Tendant home screen.
Tap and hold down the message you wish to delete. You can then select 'Yes' when asked for confirmation.
Swipe left to right on the message you wish to delete, then tap 'Delete'
It is also possible to delete multiple Voicemail messages by tapping 'Call History' on the Tendant home screen and then tapping 'Select' in the upper right. You can then tap multiple messages to highlight them and tap 'Delete' once you have selected the required messages.