Try something in these lines. I haven't compiled it, so there may be some errors.

class MyClass : public QObject
{
Q_OBJECT

private:
QLabel *label;
public:
MyClass(QObject *parent = 0)
{
label = new QLabel();
}
~MyClass(){}
void setText(QString s)
{
label->setText(s);
}


public slots:
void receiveCall(QString path, QString callNum);
}

method "receiveCall" is implemented as:

void MyClass::receiveCall(QString path, QString callNum)
{
setText("Incoming call: " + path + " " + callNum);
}

In main.cpp:

static QDBusConnection conn = QDBusConnection::systemBus();

int main(int argc, char *argv[])
{
QApplication a(argc, argv);



MyClass *mycall = new MyClass();
bool result = conn.connect("com.nokia.csd.Call", "/com/nokia/csd/call", "com.nokia.csd.Call", "Coming", mycall, SLOT(receiveCall(QString,QString)));

if(result)
{
mycall->setText("DBus connected OK!");
}
else
{
mycall->setText("DBus connected ERROR!");
}

return a.exec();
}