The IRQL_NOT_LESS_OR_EQUAL also will appear as DRIVER_IRQL_NOT_LESS_OR_EQUAL too.
Many time this is related to memory faults and errors. Since the modem is apparently old(?) maybe a PCI slot modem would be better, but let's see if we can fix yours first.
It will need to be burned to a CD or a bootable flash drive. (The DL link is well down the page.) Let it run for at least
six passes to test your memory, unless errors pop-up right away.
Since it is USB connected try uninstalling/reinstalling Drivers for the USB devices in Control Panel.
In Control Panel open Devices and Printers with the modem connected. Right click the modem icon, then trouble shoot.
A malware might be working too. Run MalwareBytes and Super Antispyware after doing updates. Do the full scans with each. Link for them in my sig.