Hello eitama,

Actually i see a mistake in the way you are trying to dial and that's the reason your operator sends error.

First off:
#31# - Hide ID
*31# - Show ID

So if my number is 0898 123 456 and my country code is 359 the number you have to dial looks like this - +359 898 123 456
When you add the code it should look like this - *31#+359898123456

I have tested the exact same way with the N900 official dialing program and it works .... doesn't work without the + before the country code.

Hope this helps.
Best regards!