Habe schon die Lösung im englischen ArchWiki gefunden.
Wiki_Archlinux.org schrieb 3G modem not detected
If NetworkManager (from v0.7.999) does not detect your 3G modem, but you still can connect using wvdial, try installing modemmanager package using pacman -S modemmanager and restart NetworkManager daemon with /etc/rc.d/networkmanager restart. Replug your modem or restart. This utility provides support for hardware not in networkmanager's default database.