Another update in my quest to get that UMTS modem work properly:
Bell technical support told me that I should run a firmware upgrade on
the stick (and gave me a link to the exact file). Of course that's a
windows .exe. I tried to run it in a second hand computer store that
let me plug in my Bell stick, but it didn't work, probably because I
didn't install the driver beforehand, the installer and upgrade
utility conflicted with each other. So I gave up for some time.
Now I've finally gotten an own computer with Windows (XP), so I
installed the modem first, then ran the firmware upgrade. Both worked
without problems. Also, on Windows the modem doesn't seem to exhibit
that disconnect problem. Then I tried again on Debian, and nothing
changed, disconnection on first try. Duh.
So, if anyone knows what's left that I could try (well I'll try
contacting some network manager people), or knows which USB UMTS modem
works on Debian in Canada (with Bell), please tell.