Hi! I had the same problem with n900. It sometimes show the telephony error, so I was restarting the phone, and error goes away. After some days it came back, and phone restart didn't helps, so i think like most people and I put some plain paper under battery on sim card, this solution works for a few hours. I consider that this is end of my phone functionality, but I look up the talk.maemo.org for any other solution and I found that fault is based on moving sim card in sim slot, so I opened my n900 put more paper under battery and sim shows up, I made some quick moves (forward backward) and sim card was not found for a while, so it's true for my phone.

What I have done to make it works:
- put plain paper under six copper flat piece of metal
- scratch with screwdriver copper place of sim card (six points)
- insert sim card with flat plain paper to sim pocket
- insert plain paper under battery

This makes sim card very stable and vibration resistant. I also want to put plastic under these six copper flat pieces but sim pocket wont close, you have to be carefull, it's very gently. I also do this http://talk.maemo.org/showpost.php?p...1&postcount=18 but with plain paper, but I think it didn't fix that.

For now it works perfect like never before, if someone wants I will do photos how it looks. Maybe this solution will help someone.