Would think, N900 could not get certified as a GSM phone without those USSD- and service-codes (call forwarding) working. I thought they are obligatory.
http://www.theunwired.net/?item=how-...ing-ussd-codes

The call forwaring applet is too simple and too cumbersome for me to use. In N95 I had call forwarding shortcuts to answering machine with 5s and 20s delays in a contacts and just pressing '3' or '5' on the keypad would dial those USSD-codes and set call forwarding service.

I also find it unbelievable Nokia does not have "a list" of functions which GSM-phone must support before it gets to prototype phase.