It stands for single line telephone also refered to as an industry standard telephone or ist. The panasonic 7000 series and 7700 series phone that the system uses are propritary and uses two pairs to communicate with the system. Slt's use one pair and are of the type that you can hook up to the CO line with out a switch. A modem like you are trying to use is a slt type device.

[Linked Image from commsmith.com]