Even if your modem was capable of that speed, the serial connection that connects it to your computer (if it's an external modem) isn't. Serial itself is too slow.
56k is an absolute maximum on an analogue phone line using audio wavelengths. Nothing faster can exist - it's been tried, and isn't reliable. In practise, dialup gives you about 4600cps max of raw transfer speed. If you get over that, it's probably modem compression. Face it, dialup uses a century old technology. Broadband is the present.
