If you have a Mac, Windows 3.11 or NT, consult your local Tech support for directions on how to do this. If not enabled, rather than changing speeds to compensate for some noise in the lines, the call will often be dropped. The Use of Data Compression or Error Detection. On some modems, having Data compression or Error Detection may cause drop outs. Try disabling modem compression and error correction in your modem initialisation string, and switching off Data Compression.
Some modems have their transmit power set too high which can distort signals.
In Australia it should be 15 dBm. Surprisingly, the new digital exchanges are less tolerant than the old ones in this respect and if the signal is too loud, they just chop off part of the signal and this can lead to the line dropping out. Many modems use the S91 and S92 registers, some modems use S61. Not all modems can alter this setting. You will need to consult your modem manual or modem supplier to find out how to adjust your hardware.