The phone line isn't REQUIRED by any means. It is recommended, however, for (as many have stated) PPV ordering.
Why? This is how I explain it to customers who call in and ask that very question:
When you order a PPV event or movie by using your remote, that information is stored on the hard drive of the receiver. The phone line is needed in this case so that the receiver can make a call, and that ordering information will be downloaded onto your billing information, so your bills will be more accurate.
If you don't have a phone line, and you order PPV via your remote, eventually, the HD in the receiver will get "full," and you'll have to connect a phone line and call DTV to have the receiver make a call to clear it.
As for calling to order PPV, here's a hint on avoiding the $5 phone transaction fee: if you go through the IVR (automated phone system), and it doesn't let you order the PPV, for God knows what reason, and you get transferred to a CSR, they'll waived that fee for ya.
There's a log on your account, though, that tells the CSR whether or not you actually were transferred from the IVR, so if you try and tell them otherwise, you'll just get transferred right back to the IVR.
And, the NFL Sunday Ticket IS like part of your programming. So no, a phone line is not required for that, either.