In order to connect at the maximum speed of 53K (phone company limit) a number of conditions need to be correct.
Your phone lines need to be in excellent condition.
A frayed cord, or bad phone can limit your connection speed.
Likewise, very old phone lines can cause problems.
The phone line between your house and the switch for your area needs to be in good condition.
Many phone lines are over 30 years old, and are affected by weather and aging.
The switch needs to be in good condition.
Many of the older switches just cannot handle the higher speed signals required for a 53K connection.
The network between your switch, and the switch that services your ISP also has to be in good condition.
The farther away the ISP office is, the greater the loss, as the signal may go through multiple switches.
Your ISP has to support 56K flex, V90, and or V92 for the high speed connection to function.
Things you can do:
Isolate your connection from the internal phone lines and see if the connection speed improves.
Complain to your phone company about the quality of the phone signal.
Contact your ISP, and see if there are any different numbers that you can use.
Also, if your modem is V92, and your ISP is V90, makesure to set the modem to V90 mode.
Chas