You get some really bad digital noise when the signal is converted from analog to digital a few times down the line. Even with the lightspan equipment, there are configuration issues on the local loops which will fowl up modems. The field techs sometimes don't know the difference between the newer and older equipment, and assume everything is LightSpan. Okay, enough geek speek, here's the english explaniation. It took me about two years of hunting through the engineers at Ameritech before we eventually figured out how everything works.
Nobody there has a complete picture of how anything really works.
People only know bits and pieces. The technology was developed a long time ago, before anyone even imagined 9600 bps modems.