I am replacing the modem. the DSL line from "Century Link" won't hook up to the CM500 modem.

[plemans]

The CM500 is a cable modem. not a dsl modem. You'd need a compatible dsl modem. 

[antinode]

> I am replacing the modem. the DSL line from "Century Link" [...]

   Why?  Does your "the modem" have a maker and model number?

> [...] won't hook up to the CM500 modem.

   I wouldn't expect DSL service to be compatible with a cable-TV-type
modem.  And if your old "the modem" was actually a modem+router, then,
even if you had a cable-TV-type ISP, you'd still need a router to use
with a (modem-only) CM500 modem.