This has been a longlived problem (several years), but I've been too busy to try to tackle it.
I have a 56K modem. I live 8 miles from the nearest telephone hub. I live in a nice suburb town outside Columbus, OH. I usually connect at about 21K (24K is as high as I've ever seen).
I have a good modem (USRobotics Sportster) and I use my high-tech employer as my ISP. Others get good speed through work.
My next-door neighbors connect to their commercial ISPs at 40K-ish speeds.
I've spoken to the phone company, but they say they won't even get involved if I connect at 19.9K or better.
I've traced the phone lines in my home to see if I could see anything that could cause me problems. I also connected a different computer -- my laptop -- to both the phone jack I use for my desktop (which I installed myself so though it might be suspect) and also several other professionally-installed jacks in the home. Same results.
My home is only 12 years old, so it should be fairly modern.
The only thing I can see is that the phone lines in the home all run through my security system box in the basement before going out to the phone company's outside box. Could that cause me a problem? If so, why? Is there anything I can do about it? By the way, I don't use the security system, so I'd actually be willing to disconnect it if necessary.
Otherwise, what else could cause me to get such drastically different results than my next-door neighbors?