The uBlox Leon G100 GSM modem has a bi-directional RESET_N line (active-low) that controls the reset of the modem but also indicates whether the modem is up or down (modem out MCU in).
Because the RESET_N line can be driven by modem only up to 1.88V we need some level shifting, like one presented in the datasheet:
Is that possible to control the RESET_N line (MCU out modem in) with the same MCU line with no impact on modem out MCU in direction? Of course, this wiring is not enough (connecting collector to ground will have no effect on the base-side).
Is the wiring below any option? It will make modem to draw current from MCU when the MCU input is in high impedance which should not be a problem.
The reason is saving 1 GPIO. I think replacing the 22k pull-up with an internal MCU's pull-up would be an option too.