The only really stupid question is the one for which an answer was never sought.
Sorry for a real stupid question,
but could this call waiting code somehow establist a third connection to an ongoing gsm voice traffic channel ?
If you are thinking of something like three-way group calling, where you have an existing call between two parties and then a 3rd caller can join in, I don't think there is any provision in GSM for such, but you can mux calls without so much trouble with a PBX like Freeswitch or a number of FOSS tools.
k/