Hello @haavard-holm
most (if not all) units have a 5 V input. Internally there often is a power regulator to 3.3 V or whatever voltage the ICs used in the unit requires. The signal lines are 3.3 V as they are connected directly to GPIOs of the ESP32 which is 3.3 V too. If the data lines of the IC in the unit have an even lower voltage level shifters are put in place.
The input voltage of the regulator is 2.7 V to 5.5 V. The minimal voltage drop between input and output is about 0.6 V for these regulators. So if you feed it with 3.3 V I'd guess the output voltage to be about 2.7 V.
According to its datasheet the SIM7020 IC has an operating voltage range from 2.1 V to 3.6V so you should be fine. Its data lines are 1.8 V which are then level shifted to the output voltage of the regulator (e.g. 2.7 V) which should still be high enough for the ESP32 GPIOs to recognize properly.
Thanks
Felix