My "favorite" thing when working tech support was explaining to people in expensive new subdivisions that Southwest Bell saved money by deploying pair gains instead of running more copper, and that's why they were never, ever going to see more than 33.6 (if very lucky) or 28.8 (more likely).

A common trick was to get them to add 3 commas to their dial string. That would prevent their modem from starting to train up until 3 seconds after they finished dialing. That would give our modems time to answer and start the 56K initiation. The delay would cause them to miss that, and then start trying to train up a much more stable 33.6 connection. It capped their max speed but made their connection a lot more reliable.

Later on, there were modems that required a CPU with MMX instructions. Dealt with that a lot when I ran my short-lived computer shop.

I made it a policy in '97 to only buy external modems.