AT command interface works! The cause of the breakage was the same Nucleus API issue with NU_Create_Timer() which we encountered at the very beginning of this project with Riviera timers: the code in uartfax.c from TCS211 was passing 0 as the initial dummy value for the timer duration, and our FreeNucleus version doesn't like it. The fix is the same: pass 1 as the initial dummy value instead.

# This configuration will eventually become a firmware image for the gtamodem
# target that provides actual GSM functionality.

target	gtamodem
feature	gsm

feature	etm
feature	tmffs2