how do i delete messages in voicemail when it says the mailbox is full?

I went to phone<voicemail, but it only gave me option to Edit (which I can only delete one message at a time) or Greeting. I don't see "deleted messages". I have a 4 and updated to IOS7.

Thanks, I was trying to avoid deleting one at a time, but went ahead and did one at a time then Cleared all. Thank you for your help.