This code:
#define PHONE_SIM_SERVICE "com.nokia.phone.SIM"
#define PHONE_SIM_PATH "/com/nokia/phone/SIM"
#define PHONE_SIM_IFACE "Phone.Sim"
#define GET_SERVICE_PROVIDER_NAME "get_service_provider_name"

	DBusGProxy *dbus_g_proxy = NULL;
	dbus_g_proxy = dbus_g_proxy_new_for_name(plugin->priv->dbus_conn, PHONE_SIM_SERVICE, PHONE_SIM_PATH, PHONE_SIM_IFACE);
	if(dbus_g_proxy)
	{
		gchar *operator_name = NULL;
		guint32 val1;
		guint32 val2;
		gint32 val3;
		if(dbus_g_proxy_call(dbus_g_proxy, GET_SERVICE_PROVIDER_NAME, NULL, G_TYPE_INVALID, G_TYPE_STRING, &operator_name, G_TYPE_UINT, &val1, G_TYPE_UINT, &val2, G_TYPE_INT, &val3))
		{
			if ((operator_name != 0) && (operator_name[0] != 0))
			{
				free(plugin->priv->operator_name);
				plugin->priv->operator_name = strdup(operator_name);
				g_free(operator_name);
				g_object_unref(dbus_g_proxy);
				return;
			}
		}
		g_object_unref(dbus_g_proxy);
	}

should be doing the same as this:
dbus-send --system --type=method_call --print-reply --dest=com.nokia.phone.SIM /com/nokia/phone/SIM Phone.Sim.get_service_provider_name

dbus_g_proxy is initialized to a valid value but the dbus_g_proxy_call call is failing