phone won't ring calls go straight to voice mail

I used to have a problem like this and it was because I did not setup voicemail at all. It would go to it even if it wasn't setup. If you go to voicemail for the first time, just setup with default message. Not sure if this is the case with you but usually that was my problem with phone going to voicemail without ringing. If you have voicemail setup, then I am not sure what it is.