can a dsl modem be used for dial up?

No, it works completely differently. In fact a DSL 'modem' is not a modem at all--'modem' is short for 'modulator-demodulator' and a DSL device works totally differently.

Dial up works using the landline (voice). DSL is a higher frequency on the same wire, so different type of modem (even though PPP and PPPoE are somewhat related). If you do connect a dial up modem to a phone line with DSL, the line to the dial up modem should have a DSL filter on it (to avoid interference between DSL and voice signals).

Note that AT&T DSL includes dial up access, but that is primarily for travel or in the rare case of DSL going down and does not use the same modem.

dsl modems are not dialup, they use a splitter to seperate the data from your voice calls, no dialup is necessay as it is always on as long as you have a connection to the central office of you local service provider (as cable pair of twisted wire). dsl modems do not have an DTMF functions to make a call out to sync up with the remote modem to download data at the really slow rates of 56k. most laptops have a fax/modem capability which you could use for dialup access, but you must be subscribed for the service and have the dialup access number.