This was also an issue for using dual SIM adapters which had to be switched to a dialed * or # code mode from STK switching mode but another phone was required which had access to the SIM Tool Kit to switch modes.
If a SIM tool kit has not been developed at this point from back when there were moderately loud calls by a real army of everyday users, and I am not sure the hardware is able to even support it, I doubt it will ever happen in the sunset years of the N900.

Though to be fair at the end of life of the Sharp Zaurus SL5500 a FOSS driver was written for the SD card reader so a complete stack of FOSS drivers made it eligible to be included in the Angstrom OS supported devices list, there were more SL-5500s produced and in circulation worldwide than any other Linux Zaurus PDA. I feel like the SD card driver was an easier job than writing a STK where NDA only encryption libs probably play a role.