Hi pusak gaoq, thanks for your answer!

But I'm afraid my problem has something to do with hardware, at least that's what the Nokia Care guy told me this morning, as he had two n900 working with Yoigo sims in his lab.

At the Yoigo shop, they told me they were positive that Yoigo doesn't have different capacity sim cads, they all are exactly the same. At least there is no sign of capacity on the sim, as I've seen in some others: 64k, 128k, 256k...

And if you read my last post, I commented that I tried 4 different Yoigo sim cards without success, and my sim works ok with another phone, so this should rule out any problems with my sim, and the other 3 Yoigo sims I have tried.

Last curious note:
- Data connection works in 3g with my Yoigo SIM!!!!

I hadn't tried that as I had to search Google and add the APN manually (curious, the Nokia Care guy told me that in his n900 this was configured automatically, but not in mine) and I never imagined data connection could work, since calls and sms don't.

This looks as a radio stack problem, but I've read that this should be fixed reflashing the phone with latest firmware, as it contains the radio.

I'm going to send it to a Nokia Service Center, as I've been suggested this morning by the Nokia support guy. Too bad, I would like to know what the exact problem is, but I'm afraid they will reflash it, test it, and replace it for another one, so I'll never know what the heck happened with my first n900.