Okay, I don't get student loans

I finished college, went into debt (because I wanted to) and have been out of college for four days. I've already gotten five phone calls about payments, made one every time, and they just keep calling back. The last one I made a payment, and three hours later they are calling again. There doesn't seem to be any rhyme or reason to it, no days I'm supposed to make a payment, just every flippin' time they call. Why isn't there simply a bill in the mail, like the household ones, followed by nasty calls if you don't make your payment? This just seems stupid, and just when I'd finally had a reasonably fun experience with the college thing.