Many customers report that they can only connect at 26,400 or lower with their 56k modem. This information will explain why this can happen and some steps to try to correct this. Consult your modem documentation to do this. Reported initial connect speeds won't necessarily be dependable or even comparable from modem to modem or location to location. If you have your modem connected to the phone line through a surge suppresser, try it without the surge suppressor.
Many surge suppressors can interfere with modem communications.
If your connections are better, you have a premises problem that you may be able to isolate and fix. If that doesn't help, listen carefully to the quality of your voice connections. Note that you must dial a known quiet number, since many otherwise good phone lines exhibit excessive noise until you actually connect. It is difficult to test for these problems without proper test equipment, but it's still a good idea to listen carefully for audible problems, particularly if you can find a number that will send you test tones. You may be able to get your phone company to improve the quality of your line.