Is this really why I could never go past 28.8k? I was 16 at that time, always wondered why - and experienced this in a few different homes until we switched to DSL.
It's very possible, especially if you lived in even a slightly rural-ish setting, like outside the city proper. There are so many reasons that could cause it, though. If you could hear any crackle or hissing during voice phone calls, that would be enough line noise to force a lower speed train-up.