Secret phone plans? No contracts? Unadvertised payment plans with no interest? These are all available. But you'll never know until you ask.
I recently decided to switch carriers to T-Mobile, so I jumped on their website to start doing the math of the different plans that they offered.
Just when I felt I couldn't possibly calculate the details of one more plan, I came across a section on the website that featured plans without contracts. This section was buried; in fact, I had to be logged on a friend's account who was already a customer to be able to see the plans at all.