I use external dialup usb modem in debian wheezy, when i plug the modem it getting /dev/ttyACM0 port, the internet connection establishes but after some hour the connection disconnect and now modem cannot establish the dialup connection to th isp with the error "alarm failed".
when i unplug the usb modem from the usb port and then plug again it start working, is there any way or script to refresh the modem without manually unplugging it?

Have you diagnosed the system log to determine if there has been any change since the modem was detected?

After the modem is installed, grab a copy of the system log:

Also check that the device is present and accessible, for instance does the modem support AT commands? Try sending AT<CR> to get a response from it.

After the problem has occurred, grab a follow-up copy of the system log via the same procedure above, but redirecting to a different file name like "modem-fail-log.txt" and see if anything is different with respect to the ACM0 device.

Re-check that the device is present and accessible, retry AT commands and see if the modem is still working.

If the modem is still working, then it may be the application or server software you're using to perform your connections. Try stopping and restarting that.