In order to get 56k, you needed to be on a reasonable quality line, and there needed to be an all digital (PRI) connection between the Telco end of your phone line and your ISP.

If your line was noisy good luck. If your line had loading coils in the path between the CO and you, that would help your voice calls but make 56k harder to achieve (and was definitely problematic for DSL). Most telephone lines had loading coils every wire mile between the CO and the customer. If you had any of the older wonky stuff telcos used to magic more pairs for customers out of thin air, chances are they wouldn't work at 56k.