Oh, you meant a literal modem-with-a-USB-port... okay, yeah. An alternative might be to use one of those standalone cellular-to-wifi hotspots, connect the iPod

Oh, you meant a literal modem-with-a-USB-port... okay, yeah. An alternative might be to use one of those standalone cellular-to-wifi hotspots, connect the iPod touch to it via wifi, and then use a VoIP app. That way you can keep the hotspot in a backpack, perhaps with a big power bank / battery, rather than dealing with the unwieldy physical dongle.