[austonianb]

Prepping a pair for ISDN could involve removing load coils (very common in more rural areas) or “designing” the pair by removing bridged taps. Both of these actions improved the quality of the facility. Also, if the facility ran through a pair gain system (or subscriber loop carrier) a reprovisioning onto a different card could be massively beneficial. I have fond recollections of the areas in rural Maine where the SLC mode kept my customers well below the maximum theoretical capability of their 56K modems, causing the normal complaints. Dialup was fun!