(Nov 14, 2003) Trying to determine whether a POTS phone line is capable of doing 56K has been a never ending challenge. 3Com/USR modems have the ATY11 command which displays the compensated frequency response of the phone line and modem.
The results can be used to determine if the phone line is 56K capable. Note that this response curve is the results of the modem adaptive equalizer’s heroic attempt to obtain the best response from a POTS phone line and not the actual frequency response of the phone line. Results will vary depending upon modem type and firmware. To find out more: http://members.cruzio.com/~jeffl/aty11/aty11.htm