Default password for voicemail?

I had to reset my iPhone, and now it keeps asking me for my voicemail password (even though I currently have no voicemail to listen to). I did not have a voicemail password before the reset, and I was able to retrieve voicemail then just fine. If there was a default voicemail password on the phone originally, I don't remember it. Does anyone know what my phone is looking for here? What is the default voicemail password, if there is one?