How to make /dev/ttyACM0 (and friends) exclusive?

Hi Everyone, I'm trying to trackdown a problem using my dialup modem. I have a program that opens the modem and watches caller id's. It flashes the hook when a telemarketer calls. It works well until... When another program opens the modem then my program starts reading intermittent responses intended for the other program. I cannot seem to open the device in exclusive mode. The current open is (I also tried with O_EXCL): int modem = open(device_path, O_RDWR | O_NOCTTY | O_SYNC); I'm not sure if it because udev configures the device incorrectly or the kernel driver is configured incorrectly. Or maybe it is impossible to do. Does anyone know how I can make the device exclusive? Thanks in advance.