The current loop is, or was, the standard way of connecting a teletype to anything, so all [Marek] had to do was construct a box that translated the signals from a GSM modem to this current loop.

The current loop idea is the base of many modern automotive sensors interfaces: Wheel speed sensors have 7/14mA, or even the more complex VDA protocol. Other sensors use PSI5, ...

Figure 4 shows the efficiency for the current-loop solution shown in Figure 2, with the 5.5-V OUT output rail tied to the BIAS pin of the LT8618.