All telephony functions, including emergency calls, are disabled due to communication error. To recover, you might have to reboot the device
You will see something like a sim card icon on the top panel when this message appears.
Awesome, it seems I couldn't make or receive call after this message is shown, I rebooted my phone and it works again ... until this week, the phone is dead, I can't use it as a phone but small tablet. Maybe I should try google to see if there's any solution and here's what I have found -
Basically the solution is to claim the warranty and Nokia replaces a new one for you, what if you are out of warranty, just someone like me? Nokia has no answer for that, thank you Nokia ;)
I was thinking "Sim card icon and communication error", maybe it is sim card slot issue? I don't know, but here's what I try -
0. Switch off N900
1. Open up N900 case at the back(battery part)
2. Take out battery
3. Take out sim card from the slot, clean it
4. Put the sim card back to the slot
5. Tighten the slot
6. Take the toilet paper, yes I say toilet paper because it was on my desk when I was trying to fix this
7. Try to tear the toilet paper and make it thicker by layering them
8. Make the toilet paper slightly same size(square) as the sim card slot
9. Put the toilet paper on top of the sim card slot and push in a bit
10. Put back your battery and press it little hard, the toilet paper will be underneath
11. Close the case
12. Switch on your phone
The phone works automagically, don't ask me why, it's really high tech fix if you ever encounter this issue.
Have fun with N900 again, by the way no fun since not much apps for it(Thank you Nokia), BUT it works as PHONE again!
p/s: By the way let me know if this solves your problem, I would like to hear about it!