The SLT Adapter converts a digital port from an ESIU PCB into an analog port which can be used for connecting on-premise 2500 type single line devices (i.e., telephones, fax machines, modems, etc.) and to telco OL13B/C OPX circuits. The SLT Adapter provides the ring generator circuit used by the analog device. The unit provides constant current which is fixed at 47 mA. Each SLT Adapter requires its own digital port.
Single Line Telephones in the Software Manual for more details.