The only ways I know of to improve your dial up connection speed are:-
Experiment with different modems. Some are better than others at coping with a bad line.
Have a word with your ISP. They may suggest you dial a different number that connects your modem to one of theirs that is more compatible.
Contact your telephone company and get them to check your line. But be warned. Firtly, you have to be persistant in your questioning. Secondly, when I did this they finally decided there was a bad connection. A few weeks later they dug up my whole street to find it. My phone (and some of my neighbours phones) ended up being disconnected for two weeks. I was not popular! Once they completed the work and everyone was reconnected, it was no better than before! I still only got about the same speed as you. But if I connected at a certain friends house I would get the full 56K (I had a laptop).
Later still I switched phone company for other reasons. Suddenly I got up to 40K+ connections regularly. Same phone line to same exchange with same number. So why it improved I have no idea.
Then I went broadband.
Best of luck.