The normal sequence for call setup is:
* Modem asks user for confirmation using icon1 / alphaId1
* User confirms
* Modem starts dialling, screen displays icon2 / alphaId2
* Call is connected, modem replies to the proactive command, screen
keeps displaying icon2 / alphaId2 until the phone call is over
According to 22.030, UDUB or CHLD=0 can only be invoked on waiting
calls. Most AT command based modems do not support using CHLD=0 on an
incoming call. So we remove the Busy method and invoke set_udub on
a call that is in the waiting state.