That's a feature I really want and it's not easy to do.
If you configure a mobile phone line without voicemail, it will not just ring and ring and ring (as I hoped it would) - instead, the caller gets a message that "the number you have dialed is not accepting calls at this time" ... or some other message (depending on the carrier) that makes it sound like you don't pay your phonebill or something.
I want my phone to ring and ring and ring until the caller gives up. I think I can do this with the twilio API, but man that's a lot of work for something (seemingly) simple.
edit: this is the discussion I found that makes me think I could accomplish this with twilio: http://stackoverflow.com/questions/22410430/twilio-respond-t...