Ok, so the solution was a piece of paper. After hours of troubleshooting, reading dead-end threads, flashing the N900 and turning it into a brick, I finally came across this post, and it saved (whatever is left of) my day. I had even spent quite some time shopping for a new phone when the paper-trick came to my attention. A piece of paper between the battery and sim where able to bring telephony back to my phone(!) Anyone searching for a solution for "Telephony Function Disabled" will hopefully start looking at this thread. And hopefully you try paper before you choose the hammer. (Well, I never tried the hammer-trick, but it seems to me to be even more scary than flashing the thing.)