Alternatively, you can probably re-format the USB-stick from the modem itself, before doing the above mount command:

mkfs.vfat /dev/sda1

Notes: There are dozens of different "filesystem types" that can be used with Linux. On a full blown PC distro, the mount command will normally automatically figure out which type a given USB-stick has been formatted with. But on an embedded device like a modem (or an empeg), fancy smarts like that are often omitted to make things smaller. And the range of supported filesystem types is also often limited to just a few. vfat (the Win98 filesystem) is pretty much universally supported though.