There are different types of DSL modems/service. The difference is how the modem utilizes the available frequency range, & the standards are not cross compatible. As long as the modem type matches, you should be OK. Typically, though, just use the one your service provider gives you. The modem will be provided by the DSL provider, not the ISP, since the ISP really doesn't care what kind of DSL you have. Typically, though, if you have the option of external or internal DSL "modems," grab the external modem.
Viper GTS