If there is less bandwidth or more noise, the modem may end up running at a lower speed. These modems are designed to adjust their speed to deal with less than ideal conditions. The additional variable is the modem itself. Some modems will deal with line conditions better than others. The telephone company cannot guarantee any minimum speed on any connection on the telephone network.
Some modems deal better with the normal limitations of the telephone network than others.
The telephone company must, however, ensure that it is providing good quality voice service. Some impairments may not be detectable with a simple voice test. The following section comes from The official 56k website. Treat this as a troubleshooting matrix. Make sure that you and your Internet service provider use the same 56K protocols.