I get this message sometimes when receiving calls on my iPhone. I do not have call forwarding on, I even turned it on and off to see if that would remedy the situation. The message seems to be innocuous, but annoying. I don't know if the two issues are related, but sometimes when dialing the iPhone and dismissing the call (or not picking up), the caller is not sent directly to voicemail, rather they are asked to reenter the phone number of the voicemail box they are trying to reach. If they do so, they get my voicemail.. but I would have to imagine this would confuse most callers.