Whenever you have hardwired (non-wireless) dsl through the analog phone lines such as verizon dsl, you must use that company's proprietary modems. I know from personal experience that that is the case at Bell South ATT and I just confirmed this at Verizon by calling and pretending to be a new customer.
Digital cable high speed hardwired internet customers have it easier. They can use those generic modems you find in places like Target or Radio Shack and are not held captive to using the company's proprietary hardware (although such hardware is typically provided).
The good news is that at least if you are a new customer or have actually damaged your modem, dsl companies such as verizon will send you one of theirs for free.