If you want to delete the actual voice mails, you have to go through your phone company (using their keypad commands) since voice mail is only accessible through your phone (but the voice mail messages are stored through your phone company provider).
Info from the Uniden DECT 6.0 6035 Manual:
Using the Voice Mail Service
If you subscribe to a voice mail service, you can use your phone to access your voice mailbox. The new message LED on the handset ?ashes and VM appears in the handset display whenever you have messages waiting in yourvoice mailbox. When you subscribe to the service, your voice mail provider provides you with the access number and the required voice mail signaling tone. You can use your phonebook to store the access number.
Resetting the Voice Message Waiting Indicator
When you receive a new voice mail message, the new message LED on the top of the handset ?ashes and
appears in the handset display. In the event your message alert tone gets out of sync with your phone company's voice messaging system, you can reset back to its original "no messages waiting" state. To reset, press and hold [?nd hs] on the base for ?ve seconds. A beep sounds and the message alert tone is reset.
Here is the manual:
Uniden 6035 1 Owner Manual