Choose Phone call from the authentication prompt (or type phone in the Second password field) and Duo will call your phone. Simply answer the call and listen to the instructions to authenticate.
Duo can text you passcodes via SMS. If you need a new batch of passcodes choose Send SMS passcodes (or type sms in the Second password field).
You can have new passcodes sent to you at any time. A new batch of passcodes will invalidate all old passcodes, so it's probably best to delete the old message when a new one comes in.