Some unnamed ISPs added the *70 or such call waiting disable to their dial up strings because of users that couldn't do that or calls to the support lines about dropped calls. I leave it to you to research that issue.

Another nutty one is the old "it worked before" issue. "Tech" runs into that and there is no cure for those owners. Even when Tech shows them it working with a new modem they won't accept that old paint has stopped performing it's old tricks. No fix for those I fear. But it makes for good income for Tech since he charges for the call in spite of the owner thinking their old modem should still work.

Sorry, no real cures to be had for free for some but I hope this gets you thinking along new lines.