[An apology: I may be committing a social blunder here, but when I first posted this message I see it ended up in the Latitude modem section. Since I'm not clear how the board works, I'm reposting it here]
Most of the times when I establish an online connection using my 600m and the PCTel 2304WT modem (driver 2.50.52) the computer appears to freeze. Sometimes if I leave it for 5 minutes or so it will come back to life. Meanwhile the modem activity lights usually show activity (though I can't identify anything running in the background).
Sometimes I have to turn the machine off and reboot to get control back.
And after I connect successfully with the modem it will work find until I connect using VPN. Then it is likely to freeze again.
I've suspected either a modem driver issue or a video driver issue, but I don't see updates available for either.
If that is the latest Dell driver, try the PCTel support site for a newer driver. Failing that, I would boot to Safe Mode and go to the Devoce Manager and delete all modem entries, then reboot and let it be found again.