I followed the current telephony code pattern in android_modem.c and reference-ril.c.

In reference-ril.c, comments say "success or failure is ignored by the upper layer here. it will call GET_CURRENT_CALLS and determine success that way." In android_modem.c, you will find similar one, i.e. the code says that success/error is ignored, the call state will be polled through +CLCC instead. So I didn't have explicit error returned.