Dear fellows out there,

I found today the solution for my problem!
I am now able to activate/cancel my call forwarding (divert) settings from command line (script).

I checked today just for curiosity the phone wiki page again (last time in September) and voila, there was an entry for one dbus command to activate forwarding calls. It was not 100% correct, but activated some forwarding and showed me the way.
Many many thanks to the unkwon contributor at the wiki.
I was so free to find out all commands and also the cancel method and edited the wiki
http://wiki.maemo.org/Phone_control#Phone

P.S.: I am almost sure there is similar command for all USSD codes ...