I have exactly the same problem (gsm network in austria).
sms incoming and outgoing is working fine with the example program.
also outgoing voice calls (example program) is ok.
incoming voice calls are not recognized by the example program.
connecting via a terminal and sending at commands:
at+cpin="xxxx"
at+clip=1
shows incoming calls - so the gsm module is working
i agree with your suspiction that the library function getVoiceCallStatus() has a bug
can anybody out in the arduino universe help and/or fix this error
thanks in advance
walter