how delete contents of voicemail

That depends on your carrier, search via Google what the rules are.

Normally you can call an abbreviated number xxxx from your iPhone. Listen to the menu.

An example here: unheard messages are stored for 8 days. Messages you've listened to will be gone after 24 hours. Longer storage is also possible. Press 5 while listening to your message and it will be saved for 7 days.