I couldn't get call forwarding, or un-forwarding to work properly on the N900 either. Call forwarding is, at least in my case (US T-Mobile), something that happens more external/on the phone company side. Phone manufacturers build in an interface that sends signals to the phone company about your call forwarding preferences and set them accordingly. Also, usually it can be an add on service/feature you have to have on your plan features.

Alternatively to the interface, the phone company may have "#" or "*" numbers you can dial to make these setting (#72 to turn on call forwarding...) but I hear the N900 has an issue with dialing numbers beginning in "#" (?).

So finally, as a last resort, I took the SIM out of my N900, put in in an old Razr, forwarded my number ("settings", "call forwarding"), saved the settings, then put the SIM back in the N900. My calls are now all forwarded and I am not messing with N900's call forwarding because I don't trust it. To un-forward, I'll return the sim to the Razr, fix it, and back to the N900.

Most Nokia Symbians I've owned had this feature under setting and it worked fine. Why an experienced phone manufacturer put a messed up one in the N900 I do not know. I do look forwarded to this being fixed on the N900 soon.