Is "call voicemail" suppose to be a command?
If you want that to work, create a contact with the first name of "Voicemail" and set the phone number to your iPhone's number.
S-
Perhaps I'm missing something, but why would you "call voicemail" on an iPhone?
I see you're in the US, so I will assume you're on AT&T which would mean you have Visual Voicemail.
Visual Voicemail downloads your messages to the phone, so there's no need to make call at all.