or are you connecting these devices directly onto the motherboard USB ports?
USB modems don't generally get on well with hubs - they (the modems) draw a fair amount of power, and are best connected directly to a USB port. This does sound like a power issue to me, although you can try one thing with the modem:
Turn off the computer. Unplug the modem from the phone socket on the wall and leave it disconnected for a minute or so. Unplug it from the USB port on the computer as well.
Now replug the modem into the phone socket, and to the computer. Reboot the computer and watch the modem lights.