hello .I don't use Linux [played with it a few times but just like windows too much I guess ]but I do use google and found this for you .Skype for Linux . http://blogs.skype.com/linux/
would need to know the make and model number of the usb modem to help you find drivers for it ,
but if the maker of the device has no Linux drivers I think there are options for youI think this is one of the reasons I don't use Linux ,too much work .lol
One assumes this is a broadband wireless modem? Current systems will have a "Mobile Broadband" connection setup option in the network manager tool. Since to the system these are just USB serial devices (modems), no special software or drivers should be required.