Because I work for an ISP I can gave you the stright dope on what goes into an internet connection, and what's happening or this side.
First of all you cannot get a 56K connetion on a normal phone line. The max is 53.3 but even that is really iffy. The reason why there is a 53.3K barrior is a little difficult to explain. I'll try my best.
You you transmit data on a phone line it a square pulse fequency wave. The higher the fequency, the faster the data can go. But also it reqires more voltage. You can only safely pump out data at 53.3K before you start blowing switches out.
Now the second thing that can make your connection slower is voltage loss on the copper line itself. When electricity goes through the line some energy is converted into heat and there is a small loss of power. Also the passing through the phyical switches at the relay points also cause loss. So the further away you are from the CO the slower the connection. Also if the main trunk is full of calls, you data has to route a longer distance through more switches to get from point A to point B.
Finaly I ask, what speed are you talking about anyway. Ire you talking about the absoulute maximum speed you can possably attain or data throughput. Both are defferant, but are related. Let us say you are connected to the internet, but sitting idle reading a page. Even though you are connected, you are not loading anything, therefore your throughput is at zero. (No data movement) So your speed is also zero. Throughput is best mesured on a graph over time as it can change. Now the maximum speed can also change as the modem can re-sync durung the connection. When you see that inital connection spped when you connect. Half the time that's not even correct and when the modem re-syncs the speed isn't updatded. It's a bug in DUN. Youre best just getting a throughput graph display for maximum speed display.
I hope that answers some questions.