Why in Ubuntu 12.04, usb-edge modem (pl2303) is not working?

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http://askubuntu.com – I use a USB EDGE modem for connecting internet in XP & Linux. Previously in Ubuntu it was detected as Prolific pl2303 converter and with WVDIAL I used internet without any problem. Recently I upgraded to 12.04 and my modem just stop working. lsusb shows the device and dmesg confirms that it is attached to ttyUSB0 but when I give command wvdial then nothing happens. A message appears "modem not responding". I did not change any settings but its just not working. What can I do now? (HowTos)