You could try a setup like this. Theoretically, it should work.
Look at the picture I've attached. Between the wall outlet and your DSL modem, there should be a filter of some sort which usually splits the line (one for the DSL modem, one for the telephone). Plug the telephone one into your dial-up modem.
I've heard of this being done, but I've never tried it myself. Like I said, theoretically, it should work fine.
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