Well, if the Yoigo SIM card works on other phones, and your N900 works with other SIM cards.. one thing that *might* explain it would be if the Yoigo SIM card for some reason needs a terminal (= phone) that supports the SIM Application Toolkit.

All phones support that, except the N900 (because it's not a phone .

The Toolkit is supposed to be only for "value-added services", but who knows what Yoigo may be doing/needing.

In any case, sorry for this suggestion: if I'm right, there's nothing you can do. If I'm wrong, you still need to identify the problem. Best of all: I have no idea how to check if I'm right or wrong

Good luck!