According to the sequence 1.2 of the ETSI TS 102 384 a busy
screen error should be returns for the display text proactive
command when the ME is not able to display the text.
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