Recently, I tried to get 3G working on my Lenovo ThinkPad X121e - it has an Ericsson F5521gw mobile broadband card. This is supported by ModemManager, but all I got were unknown errors (276 and 272).
Searching online, there were very few results (hence this short note) - just previous unrelated Linux kernel issues. I found someone with the same problem on Fedora, but no solution, so I started off by filing a bug report with Debian.
Of course, then I found the Arch user who had filed the same bug on Launchpad, and had discovered that resetting the BIOS to its default settings fixes the issue. If only that page mentioned the keywords "Ericsson", or "Lenovo"...
So after all that, it was just some weird BIOS issue. I hate hardware.